Add rmgrdesc README

Heikki Linnakangas <heikki.linnakangas@iki.fi>

Commit: f0bd0b4489fa3e53ba1612b779860d82a1b50f57
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: 2023-10-02T09:18:57Z
Releases: 17.0
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

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