Add URL wiki doc

[jaredatch]

Probably should have a doc on the Add URL feature.

Can contain when it's useful, primarily with redirects and changing the post slug.

Also can explain the checkbox to disable updates, in that it is helpful when you want to recover share counts from an old URL that now redirects, but may not want it to refresh with everything else, thus saving requests.

[billerickson]

I'll let you write that :)

I recommend posting it on the Shared Counts website as a tutorial.