README documentation cleanup

[CloudNiner]

Overview

This PR attempts to restructure the project README according to the following goals:

• Clarify the requirements and installation setup for new users
  • Simplify installation notes by providing only two user install options, either pip or from source, which is pretty standard across python projects
• Improve documentation for GPS developers
  • By grouping all information related to development into a single top level heading, rather than spread across multiple top-level headings, e.g. dev installation instructions in the installation section, which should generally be focused on user installations.
• Improve access to ReadTheDocs documentation

This PR should be considered a first draft effort. There are a few TODOs that I'd like to get input from @jbouffard about, and the sections could be restructured if we think the above goals could be met in other ways. Update: TODOs should all be removed.

The diff is pretty messy, it's likely best to review the changes by looking at the rendered view of the new document.

Notes

This PR also updates the build instructions in the Contributing docs to reference the README, since those appeared to be out of date.

I don't feel strongly about keeping the Example at the very top. Some projects do this, others start with Requirements + Installation and then show the Example. I suppose Example should stay at top if we primarily want to highlight the project's utility, otherwise swap if we think the utility is clear enough from a short intro/descriptive sentence that we can jump right to setup instructions, pushing the example down.

I signed the Eclipse CLA and commits appear verified, so I believe I have all the contributing prereqs done. Hopefully I didn't miss anything, LMK if I need to address anything else for that.

[CloudNiner]

We had also originally talked about adding a "Getting Started" section to the ReadTheDocs documentation. Rather than writing that up next, it may be worth just moving the Requirements / Installation notes changed here to the docs and just specifically linking to those sections from the README, rather than attempting to continue to maintain duplicate instruction sets in two places.

[CloudNiner]

I pushed updates for the changes we discussed just now, which include:

• Added gitter badge
• Added links section for Documentation + Gitter links at the top
• Removed getting started example from ReadTheDocs documentation
• Moved the notes about the PYTHONPATH + pip install -e . installation to "Installation for Developers"
• Removed all remaining TODOs, Justin is going to address the Geonotebook section separately