README links to docs, and docs landing page

[dbuscombe-usgs]

Currently, the link to the docs on the README goes to the page on the binary data structure.

I think a better approach would be to create a documentation "landing page" that explains the overall purpose in a concise way (perhaps use material from your proposal and paper), then links to the 3 subpages 1) raw data/export, 2) georecti, 3) binary data

[CameronBodine]

I'm closing this as documentation is now found on PING-Mapper's webpage: https://cameronbodine.github.io/PINGMapper/. Documentation will be tracked with #13.