Python runlocal should mention setting LANGUAGE_VERSION

[anowell]

Any mention of algo runlocal for python algos needs to mention that you need to first set the environment variable LANGUAGE_VERSION=python3 (or python2) for it to work.

[peckjon]

Thanks @anowell ! Moved this to https://algorithmia.atlassian.net/browse/DC-39

If you're aware of any specific locations, could you mention on DC-39? Otherwise we'll just search; should be obvious (prolly just the Py language and CLI pages)