document limitations when working with git

[Jelmer Vernooij]

It would be good to document the limitations when working with git repositories in Breezy.

E.g.:

 * No support for creating annotated tags. pad.lv/1758185
 * No support for gitattributes and the features that depend on it (lfs, eol, etc). pad.lv/1802797
 * Limited support for submodules. pad.lv/402814
 * No rename/copy inference. pad.lv/1760740
 * No support for creating shallow branches (pad.lv/1760151)
 * No good way to refer to git remotes from the command-line (pad.lv/1702283)
 * No mailmap support (pad.lv/544031)

Functionality similar to git that's missing:

 * No rebase command. pad.lv/1708046
 * No --amend option for commit. pad.lv/507529
 * No clone/fetch commands. pad.lv/831939