Add git blame ignore file and enable formatting worlfkow

Enable formatting in workflow and set git blame ignore file

Author: bkhouri

.git-blame-ignore-revs

@@ -0,0 +1,2 @@
+# Formatting code base
+19ab5e34677a7fea521b682ee35c974f7aaac0fb

.github/workflows/pull_request.yml

@@ -36,5 +36,5 @@ jobs:
       license_header_check_enabled: false
       unacceptable_language_check_enabled: false
       api_breakage_check_enabled: false
-      format_check_enabled: false
+      format_check_enabled: true
       shell_check_enabled: false

CONTRIBUTING.md

@@ -16,7 +16,7 @@ Fill the following fields:
 
 * `Title`: A one line summary of the problem you're facing.
 * `Description`: The complete description of the problem. Be specific.
-* `Expected behavior`: How you expect SwiftPM to behave. 
+* `Expected behavior`: How you expect SwiftPM to behave.
 * `Actual behavior` : What actually happens.
 * `Steps to reproduce`: Be specific, provide steps to reproduce the bug.
 * `Swift Package Manager version/commit hash` : With which version are you testing.
@@ -43,6 +43,12 @@ If you are preparing to make a contribution you should fork the repository first
 
 SwiftPM is typically built with a pre-existing version of SwiftPM present on the system, but there are multiple ways to setup your development environment:
 
+Once the repository is clone, configure your local repository so blame ignores some commits:
+
+```
+git config blame.ignoreRevsFile .git-blame-ignore-revs
+```
+
 ### Using Xcode (Easiest)
 
 1. Install Xcode from [https://developer.apple.com/xcode](https://developer.apple.com/xcode) (including betas!).
@@ -510,5 +516,5 @@ Make sure to update your TSC (Tools Support Core):
 ```bash
 $> swift package update
 ```
-Alternatively, if you are using Xcode, you can update to the latest version of all packages:  
+Alternatively, if you are using Xcode, you can update to the latest version of all packages:
 **Xcode App** > *File* > *Swift Packages* > *Update to Latest Package Versions*