doc/shortcuts: document permalink y-shortcut
You should never share link externally linking to a branch:
- line numbers may change
- the file might get renamed or removed
I did not even find the shortcut documented, even I know it existed forever as I've used it getting used to it from github.
See also:
- https://news.ycombinator.com/item?id=8046710
- https://help.github.com/en/articles/getting-permanent-links-to-files
ps: this deserves a blog post, like github has
Edited by Elan Ruusamäe