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
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b0da7ecad332244364152ff6f00fd0ceb8733781

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