Update authors and contributors list
Add initial CONTRIBUTING document

Based on
https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md

Ref #287
Remove contibuting info from docs
Replace update_lock with shutdon_event
Shut down network_load segment in tests
Allow multiple shutdowns with multiple starts
Make zsh/zpython also call .shutdown correctly
Fix update_first
Some fixes for flake8
Merge remote-tracking branch 'zyx-i/no-update_lock' into develop
Merge branch 'update-docs' into develop

TODO:

* Add info about code layout, debugging tips, etc.
* Add "developing powerline" section to docs (not contributing document)

Ref #287