Add a make rule to make it easy for users to add the git hooks.

Summary:
New users will get this automatically when they use the new
khan-dotfiles setup, but users who have already set up their machines
need to run a few commands to get the git linter/etc set up for their
systems.  This does that.

Test Plan:
Ran
   make convert
manually and saw it succeed.  Also did
   % ls -l ~/khan/webapp/.git/hooks/commit-msg
   lrwxrwxrwx 1 csilvers csilvers 51 May  1 16:58 /home/csilvers/khan/webapp/.git/hooks/commit-msg -> /home/csilvers/khan/devtools/khan-linter/githook.py*
which looks right.

Reviewers: benkomalo, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2213