Add git blame ignore file (Qiskit#1745)

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 and clang-format as our code formatting tool in Qiskit#1630 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 @@
+8c9cfb3a2cde607dd0fe3236cc5ea53e6d06b2fe

CONTRIBUTING.md

@@ -1102,3 +1102,28 @@ Now we can run the Python interpreter and pass the arguments (the python file to
     Target 0: (python) stopped.
 
 After this, you can step through the code and continue with your debug session as always.
+
+
+## Dealing with the git blame ignore list
+
+In the qiskit-aer 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.