514: Document need for req.user

[mdeggies]

Added a note under the code examples that use req.user. This section links people to 'Resolving the current user' which explains how to use the getUser middleware to access the current user account.

[coveralls]

Coverage remained the same at 68.536% when pulling 7c717b4889ae5edc57ed54f715a1b7c6a233c3d6 on 514-req-user-middleware into c64c58598697ec2acf155105bf4a207a78bcae42 on master.