Instructions to aggregate authors

Git itself offers a tool to aggregate authors in a much more versatile manner
than what we could or should provide in secondary code.

Author: uliska

README.md

@@ -85,3 +85,21 @@ An example of how to use in your templates:
 ```
 
 Alternatively, you could use the simple preformatted ``{{ git_authors_summary }}`` to insert a summary of the authors.
+
+### Aggregating Authors
+
+In some repositories authors may have committed with differing name/email combinations.
+In order to prevent the output being split it is possible to aggregate authors on
+arbitrary elements by providing a file `.mailmap` in the repository's root directory.
+This is a feature of Git itself. The following example will aggregate the contributions
+of Jane Doe committed under two email addresses:
+
+```
+# .mailmap
+Jane Doe <[email protected]> <[email protected]>
+```
+
+This will map commits made with the `private-email.com` to the company address. For more details
+and further options (e.g. mapping between different names or misspellings etc. see the
+[git-blame documentation](https://git-scm.com/docs/git-blame#_mapping_authors).
+