README: use new bug template

This will prefill all the settings so users can report things better.

Change-Id: I1ccfd3a2c6835489db1fd2ba71aee39058ffe597
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256872


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>