Add documentation about passing URL as part of contribution

[melink14]

(forked from #329)

Describe the solution you'd like
The spec mentions various contribution types which accept a URL (video, blog etc) that aren't automatically handled by the bot/CLI. In the code, we can see that contribution can be a tuple of type and URL but it doesn't seem documented. (https://github.com/all-contributors/all-contributors-cli/blob/0c7559515698b6e24919d334cd1a3713afd3a200/src/contributors/__tests__/add.js#L65)

Via experimentation I found that passing --url=<url> works since that syntax allows you to apparently add arbitrary keys to the options object.

Questions:

• Is this a feature that we want documented or is it meant to be not discoverable?
• If we want it documented, is the description above correct or is there some more canonical way to use it?
• How does it work with the bot, I couldn't see anything in the bot's code about extracting URLs so maybe it's not available there? If so it can be noted in the documentation.

After clarification I should be able to send a PR for this as well!

[Berkmann18]

Is this a feature that we want documented or is it meant to be not discoverable?

It should be documented.

If we want it documented, is the description above correct or is there some more canonical way to use it?

Not entirely sure, perhaps @jakebolam or one of the (former) maintainers who wrote that?

How does it work with the bot, I couldn't see anything in the bot's code about extracting URLs so maybe it's not available there? If so it can be noted in the documentation.

It's possibly one of the features that haven't been ported to the bot.