[CHERRY-PICK] UnitTestFrameworkPkg/ReadMe.md: Add gmock documentation

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4389

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Chris Johnson <chris.n.johnson@intel.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Oliver Smith-Denny <osde@linux.microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
(cherry picked from commit e4af282)