pgsql: rel notes: mention enabling of parallelism in PG 10

rel notes: mention enabling of parallelism in PG 10

Reported-by: Justin Pryzby

Discussion: /messages/by-id/20180525010025.GT30060@telsasoft.com

Backpatch-through: 10

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6abad00585b7d3b9bc36e10d5937c27fb6878774

Modified Files
--------------
doc/src/sgml/release-10.sgml | 11 +++++++++++
1 file changed, 11 insertions(+)