Installation docs updates

[kafitzgerald]

Several long overdue updates to the installation documentation:

• Removes outdated requirements info from the docs (it was no longer correct, depends a bit upon where it's installed from, and for most cases shouldn't be something folks need to think about when installing). In short, maintaining it seemed painful and unlikely and keeping it seemed like it was doing more harm than good at this point.
• Removes outdated info regarding installing from source. Ideally we would replace this with more up to date notes regarding the new build system, but that will probably need to come later.
• Updates NSF NCAR HPC install / usage info
• Minor updates to the README for consistency

Closes #301