README updates

[lgarrison]

I went to the README to fix the erroneous installation instructions reported by @misharash in https://github.com/manodeep/Corrfunc/issues/284 and ended up noticing some other outdated instructions. So I did a pass on the installation instructions, incorporating many of the lessons we've learned over the last few years. Hopefully it now reflects current best practices.

[manodeep]

Should all instances of pip be replaced with python -m pip? If so, then there are a couple of pytest references that need to be updated.

Separately, we should update the list of contributors to everyone that has contributed code (or create a significant contributions category and keep the list of names you have added)

[lgarrison]

In most instances it won't matter, but if the PATH gets mixed up then using python -m pip can be more robust than pip. Likewise python -m pytest can be safer.

I am open to suggestions on the contributors list! There aren't that many contributors, so we could list them all, but it seemed appropriate to acknowledge those who made major contributions/added new features.

[manodeep]

@lgarrison Is this ready to merge?

[lgarrison]

Yep! (Sorry for the delay, was offline for the last 6 days)