Add note about where members should be drawn from

Author: sethmlarson

peps/pep-0811.rst

@@ -154,7 +154,8 @@ PSRT Membership Policy
 
 The PSRT will run nominations `similar to core team nominations`_, where
 a nomination of a new member is brought to the PSRT by an existing PSRT member
-and then that nomination is voted on by existing PSRT members.
+and then that nomination is voted on by existing PSRT members. New members
+are expected to be drawn from core developers, triagers, or PSF staff.
 It is granted by receiving at least two-thirds positive votes from a vote of
 existing PSRT members that is open for one week and is not vetoed by the
 Steering Council.