Maintainers.txt: Add Package Reviewer role

Although everyone is encouraged to review patches and add their
Reviewed-by reply for a patch, with the Package Reviewer role we
identify additional community members that will be Cc'd for patches
made to a package.

A distinction between a Package Maintainer and Reviewer is that
Maintainers will always have source control push access to the package
whereas Reviewers will not. (The Reviewer may have push access if they
are also Package Maintainer for another package.)

Currently we have an limit of 2 Package Maintainers per package, but
the Package Maintainers for each package decide how to manage the
Package Reviewer list.

Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Andrew Fish <afish@apple.com>

Maintainers.txt

@@ -5,7 +5,8 @@ This file provides information about the primary maintainers for
 EDK II.
 
 In general, you should not privately email the maintainer. You should
-email the edk2-devel list, but you can also Cc the maintainer.
+email the edk2-devel list, and Cc the package maintainers and
+reviewers.
 
 Descriptions of section entries:
 
@@ -13,6 +14,8 @@ Descriptions of section entries:
      Patches and questions should be sent to the email list.
   M: Package Maintainer: Cc address for patches and questions. Responsible
      for reviewing and pushing package changes to source control.
+  R: Package Reviewer: Cc address for patches and questions. Reviewers help
+     maintainers review code, but don't have push access.
   W: Web-page with status/info
   T: SCM tree type and location.  Type is one of: git, svn.
   S: Status, one of the following: