Clarify that cargo search is not currently supported in docs

[alexjbuck]

This PR adds a single line to the README to state that cargo search is not supported by Panamax currently:

Note, Panamax does not currently support cargo search commands.

The discussion about what steps are needed to configure cargo to work with the mirror seem to indicate that once completed, cargo should function normally with the Panamax mirror. This is not wholly true, and no statement about the search functionality not being supported can leave users unclear if they have made an error, or if this is an unsupported feature.

Adding this clause to the README would remove this ambiguity for now.