Add initial documentation around themes in WSUWP

Closes #22
Add links to documentation in README

Closes #24
Add initial documentation for hooks in WSUWP

Closes #32
Add initial documentation for platform structure

Closes #33
Missed split from last commit
Describe role played by plugins in WSUWP

Starts with an initial explanation. We'll want to think about how to better expand on this over time

Closes #23
Merge pull request #34 from washingtonstateuniversity/docs-update

[PR] Documentation updates