Update readme

I stole a lot from the netcdf-c README.md.

The copyright section is a placeholder (copied from netcdf-c). I didn't find a THREDDS copyright web page or file in the repository (some but not all java source files contain copyright statement at top). Perhaps we should
1) Add a copyright.html file to the TDS and nc-j web pages
2) Add a COPYRIGHT file to repository
3) Use the various mechanisms that gitHub, I think, recently added to support license/copyright stuff.