Add documentation about passing URL as part of contribution

(forked from all-contributors/cli#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=` 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!