Add attributes information in docs

[ldecicco-USGS]

Need to add info on how to access attr(x, "request") and what you might do with it.

[ldecicco-USGS]

Also now on develop, setting:

options("dataRetrieval.attach_request" = FALSE)

and the requests won't be added as an attribute. This could be put in a script or .Renviorn or .Rprofile.