Add documentation on using specific lftp commands to Savio <-> Box doc

[aronr]

In https://github.com/ucberkeley/brc-draft-documentation/wiki/Transferring-Data-Between-Savio-and-Your-UC-Berkeley-Box-Account, add at least summary information on performing file transfers with the lftp tool.

We might at least cover the lftp commands: ls, cd, get, mget, put, and mput.

Per Wynn Meyer, lftp's mirror command is also powerful and useful:

The command to transfer a directory recursively (including everything inside it) is just 'mirror ' for downloading from box, and 'mirror -R ' for uploading to box.

Note that the mirror command might involve some element of risk: of overwriting existing files where that isn't desired. There are many options to this command - see the canonical 'man' page at http://lftp.yar.ru/lftp-man.html - and perhaps we might recommend some combination and/or best practices for minimizing that risk.

[aronr]

Some of this documentation has been added (with thanks to @smasover). It could benefit by some user testing.

Also, we might yet consider the possible addition of a (careful) mention of 'mirror', as per above.

[paciorek]

I agree mirror would be quite useful but see the point about caution. Perhaps add mention of mirror with a caveat emptor about overwriting.