Edit README documentation, add text for using Linkit with a custom link field

I see another issue for remaking the README.md file to suit template standards, so I feel this can wait until that issue is completed.

But here's an idea of what I'd like to see:

Using Linkit on a link field
To use Linkit on a custom link field, you must configure the widget on the "Manage form display" page by selecting the "Linkit" option in the dropdown.

Documentation has been added to the project page, concurrent with the 6.0.0-rc1 release, which is the first release to include this functionality.

Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.