Refactor README.md for PGXN

[mkgrgis]

Refactor to advanced FDW documentation README.md structure implemented in

• https://github.com/jeffreydwalter/redis_cluster_fdw
• https://github.com/ibarwick/firebird_fdw
• https://github.com/pgspider/sqlite_fdw
• https://github.com/pgspider/jdbc_fdw
• https://github.com/pgspider/influxdb_fdw
• https://github.com/pgspider/dynamodb_fdw
• https://github.com/pgspider/griddb_fdw

Reorder options in the main fdw file.

[mkgrgis]

Hello, @adunstan ! Have you got a time to review unified README.md about your FDW for PGXN?