Enforce formatting in CI

Update the GitHub workflows to enfore formatting, and include instructions
on how to configure `git blame` to ignore the formatting commit.

Author: bkhouri

.git-blame-ignore-revs

@@ -0,0 +1,2 @@
+# Formatting repository
+611919271c6a30badcc7d7dfedc56d4ea28740f8

.github/workflows/pull_request.yml

@@ -57,7 +57,7 @@ jobs:
     with:
       license_header_check_project_name: "Swift"
       api_breakage_check_enabled: false
-      format_check_enabled: false
+      format_check_enabled: true
 
   space-format-check:
     name: Space format check

README.md

@@ -64,6 +64,13 @@ code of conduct defined by the Contributor Covenant. This document is used
 across many open source communities, and we think it articulates our values
 well. For more, see the [Code of Conduct](https://swift.org/code-of-conduct/).
 
+Once the repository is clone, configure your local repository so blame ignores some commits:
+
+```
+git config blame.ignoreRevsFile .git-blame-ignore-revs
+```
+
+
 License
 -------
 See https://swift.org/LICENSE.txt for license information.