gitweb: update INSTALL regarding specific snapshot settings
gitweb: update INSTALL regarding specific snapshot settings

This includes instructions on how to disable a snapshot format and how
to add options to a snapshot format (e.g. setting the compression
level).

Signed-off-by: Mark Rada <156a0f7fbdb7b2cebc1c94afdf1f6f3e2a845688@uwaterloo.ca>
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