Specify why data access works

[aaronspring]

the access model is not very clear.

• nc files via opendap works only without caching

• nc files via other sources: from http(s), ftp only works with/because of caching

• http can also work with #bytes

to do: explain better in readme

[aaronspring]

Check with h5netcdf and intake xarray master

[aaronspring]

#44 #37