Update local execution instructions.

[lognaturel]

Mostly from @dorey here. The key thing is to replace the explicit install of xlrd with a run of setup.py. Additionally, I think including the virtualenv strategy explicitly is very helpful. Suggestions appreciated!

[dorey]

Looks good.

[alxndrsn]

A couple of comments on this, although I have no experience with Python:

1. these new instructions look helpful for people who are planning to make changes to pyxform, but for general use it seems much cleaner just to directly pip install (without the -e), and not worry about virtualenv. If there are reasons why that's a bad idea I'd be really interested to learn more. Otherwise, I think it would be good to move the simple, general-user setup instructions to the top of the doc.
2. on Add setup step to readme.md? #99 (comment), @ukanga linked https://stackoverflow.com/questions/19048732/python-setup-py-develop-vs-install, which suggests that pip install should be preferred over invoking setup.py directly

[lognaturel]

Hmm, good points, @alxndrsn. Typically I would say non-developers really shouldn't be coming to GitHub to get instructions but it seems like currently there isn't really anywhere else to get them from, probably because most end-users use pyxform through various wrappers. That said, there is category of people (me prior to yesterday) who want the command-line tool but don't need to modify it. So I agree with putting those instructions at the top.

[alxndrsn]

That said, there is category of people (me prior to yesterday) who want the command-line tool but don't need to modify it.

This is the category I'd choose to be in too 😄