filesystem.md - Multiple revisions

I've seen a incidents where an extension developer tries to hardcode paths
(based on their knowledge/understanding of the file-structure).  This is
usually a bad idea, so I'd like to expose the programmatic way as part of
the docs.

When we first ported to Backdrop, Jen+Nate wanted the default file structure
to be a bit different from Drupal (e.g.  `/civicrm.settings.php` instead of
`/sites/default/civicrm.settings.php`; `/modules/civicrm` instead of
`/sites/all/modules/civicrm`).

I think the Backdrop folks have a slightly different approach toward
multisite than the D7 folks: IIRC, the multisite stuff was still there
functionally, but it wasn't the default.

Separate file system doc

http://www.writethedocs.org/guide/writing/docs-principles/#cumulative
Creating a new entity should come before creating a database upgrader

spelling error

Nice documentation :) I didn't realise it existed :)

MySQL query log

CRM-20332: modify summaryAction hook to allow user-defined action in third column of action list

Correct typo - missing comma