First draft of a README for the principles/ directory. (#56)

* First draft of a README for the principles/ directory.

* More closely mirror the relevant section from the goals doc.

* Reword bit about excluding using chandlerc@'s suggested text.

* Link to goals doc.

* Update docs/project/principles/README.md

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

docs/project/principles/README.md

@@ -0,0 +1,22 @@
+# Principles
+
+<!--
+Part of the Carbon Language, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+Some language [goals](../goals.md) will have widely-applicable, high-impact, and
+sometimes non-obvious corollaries. We collect concrete language design
+principles in this directory as a way to document and clarify these. Principles
+clarify, but do not supersede, goals and priorities. Principles should be used
+as a tool in making decisions, and to clarify to contributors how descisions are
+expected to be made.
+
+A key difference between a principle and the design of a language feature is
+that a principle should inform multiple designs, whereas a feature's design is
+typically more focused on achieving a specific goal or set of goals. The
+principle can help acheive consistency across those multiple designs.
+
+Note that these principles seek to establish both the approaches the project
+wants to pursue, as well as those we want to exclude.