Add installation documentation

[bworrell]

Currently the Getting Started wiki provides details on how to install and use python-stix. Adding more docs on how to upgrade and check your current version would be helpful.

[benjaminxscott]

seems like the readme covers this pretty well?

I confirmed that the ubuntu instructions were current - added zlib since that's needed for a clean build in this branch

[gtback]

@bworrell: What else needs to be added to call this done? I think a section on determining the current version (import stix; print stix.__version__) would be helpful, either on the wiki or on readthedocs (or both).

I would really like to trim down what's in the README and point people to ReadTheDocs for more information.

[bworrell]

@bschmoker @gtback,

Thanks for taking care of this and looking things over! The updates to the README look good :) I agree with @gtback that trimming down the README would be a good idea--maybe just getting rid of the Layout section since much of that is covered by the API Reference section in RTD. I'm a bit torn on trimming out too much--I don't like having to update the same information in multiple places, but I think some of it probably should belong in the README...hmmm. Thoughts?

I think adding an Upgrading section in the Installation section of RTD would be awesome. It could cover things like checking your version via the method that @gtback mentions, and pip freeze.

We should also add a Terms section to the README. Something like https://github.com/STIXProject/stix-validator#terms