- update overview
- add tips and warnings
- add section on hook naming
- replace long code blocks (that focus more on other components) with simpler blocks that show hook usage only
- replace validate with validateForm
 - remove complicated example for setting custom values
 - update tokenValues function to match newer version
 - remove examples of executing raw SQL

This commit mostly adds Markdown tweaks.

I also struck the comparison to pre/post hooks... because I enjoy removing words. 

related to https://issues.civicrm.org/jira/browse/CRM-20187

Fixes #79

fixes #84

(the migration is easy to re-run if needed)