Clarify instructions in PULL_REQUEST_TEMPLATE.md
Clarify instructions in PULL_REQUEST_TEMPLATE.md

This change
- removes instructions about commit headers that nobody follows,
- links to useful resources for first-time contributors,
- simplifies some text, and
- hides all text inside <!--  -->.

ether/etherpad-lite,ether/etherpad-lite,ether/etherpad-lite