Add discussion of SFTP

[p-j-smith]

scp is great for automation when you know where a file is. sftp is great when you want
to manually navigate the file system before transferring files. For most people starting
with the shell, sftp might be easier to use initially as it builds on commands they are familiar with. However, this is probably a separate change. Can make a further pull request to address this. Tools such as Globus may also be worth mentioning as scp/sftp are not great for large file transfers, though Globus is not available everywhere.

Originally posted by @bkmgit in #346 (comment)

[p-j-smith]

A discussion of sftp and perhaps Globus or rsync would be nice. However, as the workshop is already too long we would like to avoid adding too much new content.

One option would be to add an extra optional lesson, like we currently have for file manipulation. If you put the lesson in the intstructors folder, then it will be available under the intstructor's view on the site (see the file manipulation lesson for an example).