Make custom connector documentation more beginner-friendly

[sirjonathan]

https://github.com/xwp/stream/wiki/Creating-a-Custom-Connector

Offer more complete start-to-finish guidance. A few specific suggestions:

• give them a complete plugin file that they can edit and drop in place (i.e. include plugin headings).
• explain, outside of code documentation, how triggering Stream works (i.e. the callback to the action)
• Offer some actual examples along with the foo / bar (i.e. wanting to trigger Stream when some action happens in a custom plugin)

[kopepasah]

@kasparsd I feel this could better be served by an example plugin repo or template repo for creating connectors. Thoughts?