Add a .git-blame-ignore-revs file to improve the output of git-blame(1) by...

Add a .git-blame-ignore-revs file to improve the output of git-blame(1) by ignoring large changes when introducing the Black source code reformatter and update the CONTRIBUTING.md guide on how to optionally use it locally.

.git-blame-ignore-revs

+21d754620fa846485fec3f4f46e8bc9d4311d1ec
+75ea5c3970d6257cacd1a196a3c05622838de738

CONTRIBUTING.md

@@ -18,6 +18,17 @@ about *Merge Requests*.
 You can also submit bugs about Debian specific issues to the Debian bug
 tracker.
 
+## Git setup
+
+diffoscope's codebase adheres to the output
+[Black](https://black.readthedocs.io/) source code reformatter. Since this was
+not always the case, the default output of `git-blame(1)` is not useful due to
+large changes made when it was adopted.
+
+As an optional step, you can ignore these commits using:
+
+    $ git config blame.ignoreRevsFile .git-blame-ignore-revs
+
 ## Testing
 
 diffoscope's test suite relies on [pytest](https://docs.pytest.org/);