Clarify Providing images for <img> how-to

[steve-chavez]

Clarify http://postgrest.org/en/v7.0.0/how-tos/providing-images-for-img.html.

Some feedback on https://gitter.im/begriffs/postgrest?at=5eccf5282c49c45f5aa197b4. Not clear that nginx sets the correct headers.

Also maybe add a note with a link to https://wiki.postgresql.org/wiki/BinaryFilesInDB. And mention CDN as an alternative for scaling, see the discussion on PostgREST/postgrest#1077.

This can be done by editing https://github.com/PostgREST/postgrest-docs/blob/master/how-tos/providing-images-for-img.rst. Click on edit, the github preview is working good.