Update the README with current information

Closes #271
Closes #274
Closes #292
auto merge of #298 : alexcrichton/cargo/readme, r=wycats

Closes #271
Closes #274
Closes #292
[WIP] Generate Cargo.lock on successful resolve
WIP
ICE
Successfully roundtrip the lockfile

This writes out and reads the lockfile, but it does nothing to verify
that the lockfile is still up to date, so it's not actually ready to be
used.
Assorted test fixes and rebase conflicts
Make Source inherit from Registry

It's infeasible to expect the central registry to be read entirely into memory
when implemented, so the list() method cannot exist. If a source implements a
Registry, however, it will not require the entire registry to be in-memory,
which should be a little more scalable.
More test fixes
Another WIP
Ensure the root's source is not in the lockfile
wip