documentation: add documentation for the bitmap format
documentation: add documentation for the bitmap format

This is the technical documentation for the JGit-compatible Bitmap v1
on-disk format.

Signed-off-by: Vicent Marti <6838a7c51796da15c3e6eae80ee6aba806e8818a@gmail.com>
Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git