MAINT: De-duplicate contributing guide

[rossbar]

Closes #286

Moves the source text for the contributing guide from site/contributing.md to CONTRIBUTING.md in the top-level directory of the repo (this ensures it will also be picked up by the GitHub machinery as well).

Then, we use the {include} directive to inject the contributing material back into the website at build time.

Finally, removes the duplicated text in the README.

NOTE: There are no content changes in this PR, just a reorganization of the existing material.

NOTE2: The first four commits are a result of copying the files in a way that preserves the git history

[bsipocz]

The myst flavour doesn't render for github markdown files.

So the alternative would be to store the file under site/; and then just link to the rendered website from CONTRIBURING.md.

Downside: the repo won't really be self contained (well, it is as the source is stored in there, too); but IMO that's OK.

[rossbar]

The myst flavour doesn't render for github markdown files.

True, but IMV that's a github problem :). In practice, the document renders okay, more-or-less largely due to the fact that ```{whatever}``` gets interpreted as just plain old ``` 's , which is good enough. Sure the admonition renders as a literal block, but these are minor considerations IMV.

I personally prefer this pattern (even with the imperfect rendering on GH) because it follows the pattern folks are used to for repositories.

[bsipocz]

OK, let's try this way. We can always switch to my suggestion if we run into problems with this approach.