pgsql: Add some comments to test_ddl_deparse and a README

Add some comments to test_ddl_deparse and a README

Per comments from Heikki Linnakangas.

Backpatch to 9.5, where this module was introduced.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8f612b7f0042a33f59cd4a18144b4d74e78b6859

Modified Files
--------------
src/test/modules/test_ddl_deparse/README           |    8 ++++++
.../modules/test_ddl_deparse/test_ddl_deparse.c    |   26 ++++++++++++++++++++
2 files changed, 34 insertions(+)