Add overview page

[kthyng]

Related to JOSS review.

Comment originally here from @chadagreene.

Sorry to bread-crumb the suggestions here, but you might also want to provide some sort overview page, where all the functions are organized in a way that users can quickly find the function they need based on the task they're trying to accomplish. I'm thinking something along these lines, and it could take the form of a documentation page accessible in the Matlab Documentation Viewer, or it could just be detailed in the README.md. Or perhaps you'll come up with some other way to do it, but you know, just something to help users figure out which functions they need for a given task.

[lnferris]

@chadagreene I have added Contents to README and am implementing documentation using relative links between markdown files e.g. https://github.com/lnferris/ocean_data_tools#building-uniform-structs-from-data-sources-1 links to https://github.com/lnferris/ocean_data_tools/blob/master/docs/argo_build.md

[lnferris]

I have added linked markdown files for all functions.