Autorest: How to write an AutoRest plugin [documentation]

[dsgouda]

@veronicagg I had written a primer on how to write a new AutoRest plugin here, I believe this could do with some more examples and explanation. Feel free to point out what specific pieces we can improve upon.

@olydis can you improve on this, outlining the new plugin model we have implemented in AutoRest so that we can have a 101 tutorial.

@vishrutshah @sarangan12

[olydis]

👍