.mergify: Add Mergify YML pull request rules configuration file
    
    https://bugzilla.tianocore.org/show_bug.cgi?id=2315
    
    Add directory for the Mergify YML configuration files that
    provides rules and actions used to process a pull request.
    
    * Auto commit a PR from EDK II Maintainer with 'push' label
      set and all CI checks pass
    * Auto close a PR from any developers without 'push' label
      set and all CI checks pass.
    * Auto close a PR from a non EDK II Maintainer that has
      the 'push' label set.
    * Post a comment to a PR that has a merge conflict.
      Submitter can resolved conflicts and reopen the PR.
    * Post a comment to a PR that fails PatchCheck.py
      Submitter can resolve PatchCheck.py issues and
      reopen the PR.
    
    Cc: Liming Gao <liming.gao@intel.com>
    Cc: Sean Brogan <sean.brogan@microsoft.com>
    Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
    Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>