Readme.md: Update EDK II CI build branch name

Readme.md: Update EDK II CI build branch name

    https://bugzilla.tianocore.org/show_bug.cgi?id=2315

Update Readme.md to indicate that that status badges are for
the edk2/master branch.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>