pgsql: Exclude snowball/libstemmer/ files from the set processed by

Log Message:
-----------
Exclude snowball/libstemmer/ files from the set processed by pgindent.
There's not much point in prettifying machine-generated code, and it
seems best to keep these files exactly like upstream anyway. Also add
some notes about why various files are excluded.

Modified Files:
--------------
pgsql/src/tools/pgindent:
README (r1.20 -> r1.21)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/pgindent/README?r1=1.20&r2=1.21)