Document bug in create-element-to-jsx, add Contributing section

[alangpierce]

As mentioned in #59, the create-element-to-jsx codemod can destroy comments,
so leave a warning in the docs so people know to be careful.

Also add a "Support and Contributing" section at the bottom of the README that
briefly describes the state and purpose of the repo and how contributions are
handled.

[cpojer]

I'd really prefer a fix in the codemod itself – are you working on this?

I'll merge this commit but am hoping for a follow-up PR.

[alangpierce]

I'm taking a look, and hoping to get something working, although it's not really urgent/blocking for me at the moment.

[reergymerej]

I may give it a go if you're not @alangpierce. Let me know so I don't step on your toes.

[alangpierce]

@reergymerej Sure, that would be great. I was mostly just investigating/learning things so far, and I probably wasn't going to continue on it until this weekend anyway.

A few things I noticed when I was initially taking a look at it, which may or may not be helpful:

• It looks like simply attaching comments to JSX nodes can cause the printer to produce code like <span>/*comment*/</span>, which isn't a valid comment. So it may be necessary to give some block comments their own jsxExpressionContainer node or something like that.
• I noticed that the JSX formatting was causing multiline React.createElement usages to be converted into single-line JSX. So it may be necessary to convert // comments to /**/ comments (which I, personally, am fine with). But I'm not really sure how all that stuff works in jscodeshift/recast.

[reergymerej]

Cool beans. Can we open an "enhancement" issue so we don't lose track of this or do we need to do that in a fork? Honestly, I'll forget all about it in a couple hours if there's no open issue. :)

[alangpierce]

@reergymerej FYI, I ended up implementing this, and sent out PR #61.