Warn about spaces in usernames.

Purpose / why

@ajstanley got stuck with Fedora rejecting data because of a space in the username.

Added to our basic user documentation.

@noahwsmith

Pull-request reviewer should ensure the following:

[ ] Does this PR link to related issues?
[ ] Does the proposed documentation align with the Islandora Documentation Style Guide?
[ ] Are the changes accurate, useful, free of typos, etc?

The person merging should ensure the following:

[ ] Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
[ ] If pages are renamed or removed, have all internal links to those pages been fixed?
[ ] If pages are added, have they been linked to or placed in the menu?
[ ] Did the PR receive at least one approval from a committer, and all issues raised have been addressed?