Fix eunit regression introduced in 2716d83a18

Use alternative way to detect net_kernel:stop/0 has succeeded as
suggested by Ulf Wiger.
Extract contributing guide into CONTRIBUTING.md

While at it, refactor the guide for clarity. Some of the new text was
taken from erlware/relx/CONTRIBUTING.md and modified as needed.
Always return the errors and warnings from erlydtl

This allows us to use `rebar_base_compiler:ok_tuple/3` and
`rebar_base_compiler:error_tuple/5` when returning from
`rebar_erlydtl_compiler`.
Merge pull request #109 from tuncer/contributing

Extract contributing guide into CONTRIBUTING.md
Merge pull request #122 from tuncer/net_kernel

Fix eunit regression introduced in 2716d83a18
Merge pull request #111 from tuncer/nox-erlydtl-compile

Always return the errors and warnings from erlydtl