docs/process: branching-checklist: Add a list of accounts at the beginning

The checklist requires to have access to several accounts on both
xenbits and osstest. List those accounts at the beginning of the file
so it is easier to check if one has the permissions before starting
the branching process.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>

----
    Changes in v2:
        - Add Henry's reviewed-by

docs/process/branching-checklist.txt

@@ -1,3 +1,11 @@
+=== Before starting ===
+
+Access to the following accounts is necessary:
+    * xenbits.xen.org: xen, xendocs
+    * osstest.osstest-test-lab: osstest
+
+=== Check list ===
+
 v=4.1
 ov=4.0