Update mmgr's README to mention BumpContext
Oversight in
29f6a959c.
In passing, since we now have 4 memory context types to choose from,
provide a brief overview of the specialities of each memory context
type.
Reported-by: Amul Sul
Author: Amul Sul, David Rowley
Discussion: https://postgr.es/m/CAAJ_b94U2s9nHh--DEK=sPEZUQ+x7vQJ7529fF8UAH97QJ9NXg@mail.gmail.com