README improvements

[apreucil]

A few notes based on my experience creating a new analysis script based on available documentation. This is based on the update-readme branch.

1. I think the word "plugin" is a bit off-putting and perhaps even scary to someone who only casually develops in python. Maybe rephrase this or define what is meant by plugin in the context of GFDL (i.e., a simple way to gain access to analyses that someone else has written).
2. In the section for Analysis Script Developers, it may be necessary to emphasize the importance of creating the package and using class inheritance. This should also include definitions of classes and packages in a friendly way that is very inviting to someone who's never used these types of tools before.
3. The README is currently unclear as to what file to put the class in. Inexperienced users would benefit greatly from being explicitly told where to put analysis code. Users who are more experienced may be able to take advantage of the framework to create more organized packages.
4. The "requires" function is intimidating such that I didn't even try to use it. We need to explain this function in a way that is easier on the users to build?
5. A contribution guide should be added for users not as experienced with git (i.e., please fork the repo, create a branch, add your code, test, and submit a pull request). Include links to GitHub how to's where appropriate.