create contributing guidance

[MathewBiddle]

Related to #426, but we should create CONTRIBUTING.md to help guide folks how to contribute.

[MathewBiddle]

woops, looks like I put that in the wiki. What's better?

[lohithmunakala]

Keeping it inside the website itself might be better as per my opinion.

[ocefpaf]

Keeping it inside the website itself might be better as per my opinion.

+1 to both actually. The website is markdown and there is no reason why not using the same file as the GiHub "standard" CONTRIBUTING.md and a page in the live site.

[lohithmunakala]

If we add this to the website, which chapter should I add it to?

[MathewBiddle]

Add another chapter under OTHER RESOURCES for Contribute to IOOS CodeLab.

[MathewBiddle]

fixed with ioos/ioos_code_lab#20