Try-Us-Out instructions should not use Docker, but binaries about docs

[tbg]

I think we should start exposing our linux nightlies and the brew formula (assuming it works fairly reliably). We can keep the Docker stuff around somewhere, but maybe not at the top.

I'll fix up the README invocation. Should be passing a flag or two, but on OSX there's always some loss of simplicity because of the VM.

[jseldess]

Fixed with readme updates: cockroachdb/cockroach#5511. We're now just linking to our install page in the docs.