pgsql: Add rmgrdesc README

Add rmgrdesc README

In the README, briefly explain what rmgrdesc functions are, and why
they are in a separate directory. Commit c03c2eae0a added some
guidelines on the preferred output format; move that to the README
too.

Reviewed-by: Melanie Plageman, Peter Geoghegan
Discussion: https://www.postgresql.org/message-id/9159daf7-f42d-781b-458f-1b2cf32cb256%40iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f0bd0b4489fa3e53ba1612b779860d82a1b50f57

Modified Files
--------------
src/backend/access/rmgrdesc/README           | 61 ++++++++++++++++++++++++++++
src/backend/access/rmgrdesc/rmgrdesc_utils.c |  4 ++
src/include/access/rmgrdesc_utils.h          | 44 --------------------
3 files changed, 65 insertions(+), 44 deletions(-)