Add a "How to say things" section to CONTRIBUTING (#2715)

Incorporates some content from [this document drafting text on how to contribute effectively](https://docs.google.com/document/d/18DY8KSp7czJy7pDfIELwf1hAXE5tIvCwfBIQufaSSQY/edit?resourcekey=0-ATblx6fYlWDu6oMpaL5NNQ#heading=h.q2grb4tvzv0c).

Co-authored-by: Adrien Leravat <Pixep@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Geoff Romer <gromer@google.com>

CONTRIBUTING.md

@@ -28,6 +28,8 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
     -   [Contribution tools](#contribution-tools)
     -   [Contribution guidelines and standards](#contribution-guidelines-and-standards)
         -   [Guidelines and philosophy for contributions](#guidelines-and-philosophy-for-contributions)
+        -   [How to say things](#how-to-say-things)
+            -   [Make your point concisely](#make-your-point-concisely)
 -   [Style](#style)
     -   [Google Docs and Markdown](#google-docs-and-markdown)
     -   [Other files](#other-files)
@@ -73,8 +75,9 @@ has questions the community is looking for a decision on. The
 will have more mature proposals that are nearing a decision. For more about the
 difference, see the [evolution process](docs/project/evolution.md).
 
-When giving feedback, please keep comments positive and constructive. Our goal
-is to use community discussion to improve proposals and assist authors.
+When giving feedback, please keep comments positive, constructive, and
+[concise](#make-your-point-concisely). Our goal is to use community discussion
+to improve proposals and assist authors.
 
 #### Contribute design ideas to Carbon
 
@@ -296,6 +299,78 @@ follow the Carbon documentation and coding styles.
     -   Keep in mind that code contribution guidelines are incomplete while we
         start work on Carbon, and may change later.
 
+#### How to say things
+
+-   Treat others with respect.
+-   Recognize that other points of view are valid, and most decisions are about
+    choosing the best set of trade-offs or closest alignment with Carbon's goals
+    since there usually isn't a single best answer. It is important to frame
+    feedback and discussion about someone else's proposal under the assumption
+    that they too have deep experience in the area but may have come to a
+    different conclusion.
+-   Be clear when something is an opinion by using "I" or "me":
+    -   Not as helpful: "`foo` is objectively better word to use"
+    -   More helpful: "I find `bar` confusing since it has this alternate
+        meaning, I think `foo` is clearer."
+-   It can be helpful to define your terms.
+
+When trying to make a point, please employ these strategies to make your
+argument _effective_ and _helpful_:
+
+-   Focus should be on explaining the basis by which others can come to a
+    conclusion. Generally this means connecting a potential solution to the use
+    cases it helps with.
+-   Be specific and concrete, using examples to demonstrate the benefits and
+    disadvantages of the different options
+    -   Minimally helpful: "I like <X>", "Carbon should have feature <Y>"
+    -   More helpful: "I think Carbon should have feature <Z>, it would mean
+        <this example> would be written <like this> instead of <like
+        that>." (Assuming the reader will think the example is representative of
+        an important class of use cases.)
+    -   Very helpful: "If we go with approach <X>, it helps with problem
+        <P> in <this example> of use case <A>. It doesn't help with use
+        case <B>, but that can be better solved by <Y>, as can be seen in
+        <other example>."
+    -   Very helpful: "Yes <X> gives a nice answer in that case, but it has a
+        problem / I don't see how it applies in <this other situation>."
+-   Explain the reasoning by which you can come to your conclusion
+    -   Avoid [fallacies](https://en.wikipedia.org/wiki/List_of_fallacies) like
+        [arguing from authority](https://en.wikipedia.org/wiki/Argument_from_authority)
+    -   Don't expect people to read long works to understand your point
+
+If someone questions or argues with your point, try to directly address the
+points being made. Try not to step backwards or switch to a more general or more
+meta level as that can seem like you're evading the question.
+
+##### Make your point concisely
+
+-   **Asking questions is OK**: Asking questions about a particular discussion
+    is almost always OK. If you're worried the questions might be more about
+    _background_ and might be long enough to get distracting, you can always ask
+    them in some of our dedicated spaces like `#language-questions` on Discord.
+-   **Favor new and relevant information**: When sharing ideas and opinions in
+    community discussions, it is important to do so in a way that both _adds new
+    information_ in some way/shape/form, makes sure that information is
+    _relevant_ to the discussion, and avoids _repetition_. This means reviewing
+    what you write before posting a larger response, and editing it down to just
+    the points you want to make that have not already been made, and any new
+    arguments supporting those points.
+-   **Be inclusive by staying concise**: We need to be mindful that writing many
+    paragraphs of extra text is going to exclude people. Some people can be
+    excluded from consuming the conversation, either because a wall of text is
+    too intimidating to read, or they don't have the time or bandwidth to wade
+    through the extra text to find the new information being conveyed. It can
+    also drown out other contributors
+-   **Prefer upvote to repetition**: One person saying "I don't like this
+    feature" is useful, and that same message with 100 upvotes is extremely
+    useful, but 100 people writing separate messages saying "I don't like this
+    feature" is not. Emoji reactions are available in both Discord and GitHub
+    and should be used for this purpose in both.
+
+It is also okay to do things like ask a question to get clarification about what
+someone has said or to solicit opinions about various options. Just be
+respectful, and don't drown out other discussion.
+
 ## Style
 
 ### Google Docs and Markdown