cargo add documentation missing some flags

[ehuss]

There are a few flags missing from the documentation of cargo add:

  -p, --package <SPEC>        Package to modify
      --offline               Run without accessing the network
      --dry-run               Don't actually write the manifest

These should probably be added to those docs. I think --package and --offline have stock includes. There are instructions at https://github.com/rust-lang/cargo/tree/master/src/doc#building-the-man-pages to rebuild the documentation after modifying the linked file.