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 <jdblischak@uchicago.edu>
Merge pull request #15 from wking/r-instructions

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