maint: store git master rockspec file on git master branch.

It's really easy for git master to change incompatibly with the
specl-git-1.rockspec from the previous release, so maintain and
advertise it on its own branch.
* .gitignore: Don't ignore specl-git-1.rockspec.
* specl-git-1.rockspec: Save to version control.
* README.md: Adjust git rockspec URL.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>