* Move How to Contribute to documentation and update it

* Consistent formatting

I think if we do another one of these it'd be good to turn this into a list, but this is good for now.

Thanks @spicyj!
cc @gaearon

took out quotation marks from CLA header as per MLA standards

Make the contributing sentence make a little bit more sense.

Update links to use https:// where it is supported. There's probably a lot
more that could be fixed, but these are the core ones I found (especially
the download links in order to prevent MITM attacks). Note that there are
some fb.me links that will redirect to http:// even while accessed over
https://, but this seemed like the best way to fix those for now.

NOTE: Only non-third-party files were modified. There are references to
http:// URLs in vendored/third-party files, but seems appropriate to fix
upstream for those rather than editing the files.

Also, copy one image locally to the blog, as it was hotlinking to a site
that did not support https://.

Last, use youtube-nocookie.com instead of youtube.com for video embeds,
as the former doesn't try to set a cookie on load (privacy enhancement).

The line above it recommends `'` over `"`, and yet the line in question uses `"`.

Updated the mention of "facebook.com" on line 3 of the CONTRIBUTING.md file to link to Facebook. Previously, this mention was just in plain text.

Changed from `facebook.com` to `[facebook.com](https://facebook.com)`

No other content has been changed. Content has not been removed from this file in any way.

The use of the conjunction "and" leads to an improper assertion about what should/shouldn't be done.  Using a semicolon resolves this by indicating the contrasting alternative.

Updating doc to point to the brand new CLA form at https://code.facebook.com/cla

The documentation states that React.renderComponentToString
'uses a callback API to keep the API async', but the
implementation is actually synchronous. In order to maintain
this contract the callback should always be called
asynchronously or be change to a synchronous API.

As per the discussion of pull request 982, the API should
be changed to a synchronous one.