Ensure all plugins have a CONTRIBUTING.md file

[mbbroberg]

From previous work:

A user want to start contributing to sensu, what do they do?
Below is a rough start on documenting all that is needed.

Getting Started

Developer Guidelines

Testing Process

I'd like to take this work, merge into something we want to use and then use a tool to open PRs across all our plugins (I have one I like for that). So this issue would be to:

• Create a decent, default CONTRIBUTING file for a plugin
• Link it here

And that's it - from there I'll take it and distribute to all other plugins. Start wherever you'd like 😄

[mbbroberg]

Contributing can emulate some of the workflow layout work that Kubernetes has perfectly done here: https://github.com/kubernetes/community/blob/master/contributors/devel/development.md

Having that in each contrib file will keep people informed on how to manage their forks and thus reduce the chance of rebasing issues.

[mbbroberg]

Another example of this done well: https://github.com/jekyll/jekyll/blob/master/docs/_docs/contributing.md

[mbbroberg]

Merged. Will close this issue after distributing it: https://github.com/sensu-plugins/community/blob/master/templates/CONTRIBUTING.md