Add codeowners

CODEOWNERS

+# This is a comment.
+# Each line is a file pattern followed by one or more owners.
+
+# These owners will be the default owners for everything in
+# the repo. Unless a later match takes precedence, they
+# will be requested for review when someone opens a pull request.
+*       @rboman
+
+# These owners own a directory and nested subdirectories
+/fdtd/ @rboman
+/flow/ @acrovato
+/heat/ @rboman
+/mirrors/ @kliegeois
+/mrstlnos/ @kliegeois
+/sph/ @rboman
+/tlnos/ @rboman
+/waves/ @rboman
+
+# You can also use email addresses if you prefer. They'll be
+# used to look up users just like we do for commit author
+# emails.
+#*.go docs@example.com
+