Add git blame ignore file

This commit adds a new file with the SHA1 of commits to ignore when
running git blame. This is important because of the recent adoption of
black as our code formatting tool in Qiskit#6361 we caused a large amount of
code churn to change the code formatting. However using the ignore file
is a local opt-in feature for git and not something we can enable
globally by default. To facilitate this a section is added to the bottom
of the contributing guide to document how this file can be used.

.git-blame-ignore-revs

@@ -0,0 +1 @@
+2eee56616d50a9e26756f855ef4aa0135920ad78

CONTRIBUTING.md

@@ -422,3 +422,28 @@ the following steps:
 
 The `stable/*` branches should only receive changes in the form of bug
 fixes.
+
+## Dealing with the git blame ignore list
+
+In the qiskit-terra repository we maintain a list of commits for git blame
+to ignore. This is mostly commits that are code style changes that don't
+change the functionality but just change the code formatting (for example,
+when we migrated to use black for code formatting). This file,
+`.git-blame-ignore-revs` just contains a list of commit SHA1s you can tell git
+to ignore when using the `git blame` command. This can be done one time
+with something like
+
+```
+git blame --ignore-revs-file .git-blame-ignore-revs qiskit/version.py
+
+```
+
+from the root of the repository. If you'd like to enable this by default you
+can update your local repository's configuration with:
+
+```
+git config blame.ignoreRevsFile .git-blame-ignore-revs
+```
+
+which will update your local repositories configuration to use the ignore list
+by default.