.mergify: Add Mergify YML pull request rules configuration file

.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>