Add user guide

[tomlunderwood]

I converted the user guide for this tool which was on Confluence (at https://stfc.atlassian.net/wiki/spaces/P/pages/865370176/User+Guide+for+the+Data+Transfer+Tools+Container+Toolbox) into markdown and added it to this repository as USERGUIDE.md. Moreover, I modified the README.md to reflect this change.

Note that the user guide (even the Confluence version) still needs some work done, in particular the 'Using the container: Other Available Data Transfer Tools and Commands' section. I have highlighted this in the user guide.

This PR closes Jira issue https://stfc.atlassian.net/browse/PSDI-446

[amali-pawula]

This line needs to be displayed like this --> <file path to upload> -> ex: /data/<file name> but in the document I can see like this --> * -> ex: /data/ (with out <file path to upload>)

Maybe we have to use the " ` " symbol rather than the " * "symbol

This needs to apply to all other places as well 😁

[tomlunderwood]

I've amended the text significantly in response to this. In fact, I've done more than you asked. I've added a little more text around the commands which had <file path to upload> -> ex: /data/<file name> to help it read better. Moreover I've changed, e.g. <file name> and <file path to upload>, so that it reflects the conventions given earlier in the document, e.g. that the directory on the host is <host-data-dir>.

If you are not happy with my changes then note that the section Using the container: Other Available Data Transfer Tools and Commands will have to be looked at again anyway- to double-check that the commands are correct (after my changes). I will make a Jira ticket to do this. Since it will be looked at again for this ticket, there will be the opportunity to change anything you don't like about it, including anything I have done in this PR.

[amali-pawula]

Thank you for these modifications, Tom. I like these changes, and I don't have any objections regarding these changes. So I'm approving this PR now.