Add Michael and Mark to committers list

The OpenTitan Technical Committee is happy to announce that Mark
Branstad and Michael Munday are now committers!

Thanks for your great work on OpenTitan so far, we're looking forward to
more of it!

Signed-off-by: Philipp Wagner <phw@lowrisc.org>

COMMITTERS

diff --git a/COMMITTERS b/COMMITTERS
index 0418278..0493de9 100644
--- a/COMMITTERS
+++ b/COMMITTERS
@@ -7,6 +7,7 @@
 
 Committer list:
 * Alex Bradbury (asb)
+* Mark Branstad (mwbranstad)
 * Greg Chadwick (gregac)
 * Cindy Chen (cindychip)
 * Timothy Chen (tjaychen)
@@ -17,6 +18,7 @@
 * Martin Lueker-Boden (martin-lueker)
 * Rasmus Madsen (rasmus-madsen)
 * Felix Miller (felixmiller)
+* Michael Munday (mundaym)
 * Miguel Osorio (moidx)
 * Dominic Rizzo (domrizz0)
 * Tom Roberts (tomroberts-lowrisc)