Update README file extensions to .rst

[taycaldwell]

Updated the file extensions of the README files in the repo to have the .rst file extension so they are rendered properly on GitHub.

[ghost]

Not all of them should be a simple rename, see if you can bring .rst formatting in the mix.

[refi64]

If you want, you can also use some of the stuff I did in #5

[taycaldwell]

@rogersachan Files have now been fixed and updated with proper .rst formatting with respect to how GitHub renders them. A lot of the existing files used proper syntax and looked fine. For those files I just added the file extension so they could be rendered properly.

The biggest formatting changes were applied to the PCBuild README.

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[brettcannon]

Would it be possible to break this PR up into separate ones per file? I know that might be a pain but it would make it easier to review since e.g. I don't know if @zooba and @zware actually want to the READMEs related to Windows builds to change (plus you stripped out Windows line endings in those files).

[brettcannon]

GH-104 renames Doc/README.rst as well.

[taycaldwell]

@brettcannon I'm fine with breaking it down into multiple PRs.

[zooba]

I'm not concerned about this one (though the other readme "fix" broke our release builds...). Ideally we'd have a .txt version there too so that it's easy to read the file from a clone/tarball, but if the GitHub masses would prefer .rst then I'd rather only have one copy. It's hard enough to keep it up to date already...

[brettcannon]

OK, then let's do this file-by-file as there's so many PRs flying by that are grabbing this sort of low-hanging fruit it's probably best to do this slowly and methodically so that @taycaldwell doesn't have to constantly rebase because someone did another README.