docs: separate custom query documentation into a new file

This MR improves the wording of custom query documentation and extracts the documentation into a separate file.

How to review

This MR is separated into two commits, one just moves custom query part of the README into a separate file and the second changes the wording of the custom query documentation. The best way to review the wording is to only look at the second commit

/cc @aqualls I've created an MR as you asked me (#336 (comment 550978331))

/cc @jboyson since you were participating on the issue

Related to #336 (closed)