github: Add CODEOWNERS file

Add initial CODEOWNERS file that sets the @pmodels/mpich-intel as owner
for collective frameworks initially contributed by them. The idea is
that pull requests to this code automatically request a review from the
owner.

The intention is for code owners to have the authority to accept or
reject changes based on whatever criteria they like, with some
exceptions. For example, security or licensing issues may result in
changes to the code by the core MPICH team without involvement of the
code owner.

.github/CODEOWNERS

@@ -0,0 +1,4 @@
+# The Intel MPICH team is primarily responsible for the collective TSP
+# and algorithm frameworks.
+/src/mpi/coll/algorithms/ @pmodels/mpich-intel
+/src/mpi/coll/transports/ @pmodels/mpich-intel