Add 12_repos.ipynb notebook

The list of repositories comes from the 'gen-test' repository by Derrick
Stolle (https://github.com/derrickstolee/gen-test), which repository was
originally used to benchmark different proposals for generation number
v2 in Git.

Includes summary of repositories statistics - the largest repository
is Linux, with Chromium a close second.

Author: jnareb

12_repos.ipynb

@@ -63,6 +63,7 @@ Only the first command performs straight reachability query.
 9. [Extracting commit graphs from Git repositories](09_git.ipynb)
 10. [Checkpointing](10_checkpoint.ipynb)
 11. [Graph datasets](11_datasets.ipynb)
+12. [Large Git repositories](12_repos.ipynb)
 
 ## Slides