README.md: Mention that R should be installed before running our installer
README.md: Mention that R should be installed before running our installer

This came up in response to the v0.1 release announcement on
discuss@lists.software-carpentry.org [1].

I've also added a link to Python (and used it where we mention Python)
to match the existing link to R.

[1]: http://lists.software-carpentry.org/pipermail/discuss_lists.software-carpentry.org/2014-August/001932.html

Reported-by: John Blischak <5b232e11387ace3f316588355155f2f2e2929cc5@uchicago.edu>

wking/swc-windows-installer,ethanwhite/windows-installer,swcarpentry/windows-installer