Create decision for proposal 149--Change markdown style guide (#165)

* Create p0149_decision.md

Create decision for proposal 149--Change markdown style guide

* Update proposals/p0149_decision.md

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

* Update proposals/p0149_decision.md

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

* Run pre-commit

* Update README.md

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

proposals/README.md

@@ -43,5 +43,6 @@ request:
 -   [0143 - Numeric literals](p0143.md)
     -   [Decision](p0143_decision.md)
 -   [0149 - Change documentation style guide](p0149.md)
+    -   [Decision](p0149_decision.md)
 
 <!-- endproposals -->

proposals/p0149_decision.md

@@ -0,0 +1,51 @@
+# Decision for: Change markdown style guide
+
+<!--
+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 2020-09-22
+
+Affirming:
+
+-   [chandlerc](https://github.com/chandlerc)
+-   [geoffromer](https://github.com/geoffromer)
+-   [noncombatant](https://github.com/noncombatant)
+-   [tituswinters](https://github.com/tituswinters)
+-   [zygoloid](https://github.com/zygoloid)
+
+Abstaining:
+
+-   [austern](https://github.com/austern)
+-   [josh11b](https://github.com/josh11b)
+-   [gribozavr](https://github.com/gribozavr)
+
+## Open questions
+
+### Which style guide?
+
+The core team decided to let the Painter choose the style guide to be used. The
+painter selected to continue using the Google documentation markdown style
+guide.
+
+## Rationale
+
+The core team reached consensus that there should be a style guide, but did not
+have a strong preference between the Google style guide and the Microsoft style
+guide. One advantage of the Microsoft style guide is that it is less
+Google-centric, which could contribute somewhat to our community goal. Given the
+level of experience of the core team members with these style guides, it is not
+clear that a well-informed decision could be made. As such, the core team
+decided to leave the decision of the specific style guide up to the Painter.
+
+### Painter rationale
+
+Given the lack of a high level reason to pick one over the other, the painter
+looked at the available tooling for linting against the two options and spoke
+with people who have written substantial developer documentation against both
+style guides. There was no persuasive reason to change from the Google
+documentation style guide at this time. Specific ambiguities or problems with
+that style guide should instead be addressed, if necessary at all, with
+Carbon-specific local rule.