Document performance considerations

[wolfgangwalther]

This could also go into the "howto issue", but I think the performance topic deserves a separate issue. We might need to invent a new section somewhere in the docs - or just write a "How to make PostgREST fast".

Old postgrest-docs issues about this:

• Remove and warn about STRICT in functions postgrest-docs#224
• Recommend to always label the functions with the strictest volatility postgrest-docs#255
• current_setting can lead to bad performance when used on RLS postgrest-docs#609

But there's probably more to add - so this is an invitation to chime in with more tips and tricks about how to make PostgREST run fast.