Move rationale to the proposal template (#459)

In support of #426. The decisions need to be merged into proposals, but I want to handle that separately.

proposals/template.md

@@ -17,6 +17,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 -   [Background](#background)
 -   [Proposal](#proposal)
 -   [Details](#details)
+-   [Rationale based on Carbon's goals](#rationale-based-on-carbons-goals)
 -   [Alternatives considered](#alternatives-considered)
 
 <!-- tocstop -->
@@ -56,6 +57,13 @@ will that in fact solve it?
 
 TODO: Fully explain the details of the proposed solution.
 
+## Rationale based on Carbon's goals
+
+TODO: How does this proposal effectively advance Carbon's goals? Rather than
+re-stating the full motivation, this should connect that motivation back to
+Carbon's stated goals for the project or language. This may evolve during
+review.
+
 ## Alternatives considered
 
 TODO: What alternative solutions have you considered?

proposals/template_decision.md

@@ -1,32 +0,0 @@
-# Decision for: TODO
-
-<!--
-Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-Exceptions. See /LICENSE for license information.
-SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
--->
-
-Proposal accepted on 2021-MM-DD
-
-Affirming:
-
--   [austern](https://github.com/austern)
--   [chandlerc](https://github.com/chandlerc)
--   [geoffromer](https://github.com/geoffromer)
--   [gribozavr](https://github.com/gribozavr)
--   [josh11b](https://github.com/josh11b)
--   [noncombatant](https://github.com/noncombatant)
--   [tituswinters](https://github.com/tituswinters)
--   [zygoloid](https://github.com/zygoloid)
-
-Abstaining:
-
-## Open questions
-
-### TODO question?
-
-TODO answer.
-
-## Rationale
-
--   TODO