DOC: Add warning for newbs not to edit auto-generated file
@jreback, here is the proposed changed we talked about in [this PR](https://github.com/pydata/pandas/pull/10337#issuecomment-111822584) for other fellow newbs that make the same mistake I did.

You said "and more importantly prob a note in `internals.rst` about how/what to change", but I didn't understand that.  `internals.rst` is not auto-generated, right?  So, why would I add a note there?  Perhaps you wanted a note to appear in `internals.html`?  That seems unnecessary since if the user has that file on their machine, they must have intentionally generated it...

This is where `internals.rst` ends up in the generated docs: http://pandas.pydata.org/pandas-docs/stable/internals.html

That said, I think the warning in the generated file serves this purpose more directly. Warnings in docs tend to get ignored anyways :).
 @shoyer, hmmm.. maybe I'm confused.  So are you saying it would be good to have a comment in `internals.html` warning new devs that `internals.html` is auto-generated and they should edit `internals.rst` instead?  (similar to what I just did for `generated.pyx`)

If that's the case, we should probably have a similar comment in all the [generated] HTML files, right?
 @Garrett-R No, I think @jreback was thinking you could have the warning "generated.pyx is generated from make_generated.py" in the internals documentation. None of the documentation HTML pages are checked into git, so there's no danger we could get a pull request editing them directly.

HOWEVER, I actually disagree with @jreback on this one. I think the warning you have here is sufficient.
 Ah, I see.  Yeah, I agree with you.  I think having the source code warning in `generated.pyx` should be enough.  It's already kind of embarrassing that I didn't realize that `generated.pyx` was generated.
 no the issue is that I don't think it's obvious that you need to Python generate_code.py

that instruction should live maybe in the contributing docs as well as in generated.pyx 
 Oh yeah, that's true, it's definitely not obvious that you need to run `python2 generatre_code.py` (I had to be told).  

The only situation you'd ever need to know this is if you're actually trying to modify `generated.pyx`, so one option would be to just include it these instructions in both `generated.pyx` and `generate_code.py` (which I've now done), but not add extra noise to the contributing docs.
 hmm this should be compat but I guess that's another issue

ok then on the py2
 Yeah, it would be nice to have it compatible.
 merged via https://github.com/pydata/pandas/commit/758ca05e2eb04532b5d78331ba87c291038e2c61

thanks!
