Clarify How Custom Instances Are Configured

[Airradda]

It would be help if it clarified in the README or the configuration popup's field, Custom Instance (Optional), what format the instance URL is supposed to be in. I was about ready to switch back to Libredirect as the addon was ignoring my custom instances and directing me to the same rate limited ones. It took me reading through the code to find that it should be example.com with no prefix or trailing slash and so you can't simply copy paste from the URL bar or a link.

[libreom]

Thanks for feedback, This is needed, what should be written in your opinion? For eg- "Custom Instance (eg- nitter.net)" or anything else. I'll clarify first in README, then in input area, and will also add validation and error handling it it's not written the right way

[Airradda]

Something like "Custom Instance (eg- nitter.net)" would work since I think "Custom" indicates it being optional pretty well.

[libreom]

Already Completed with 3ced4fc It has not been released and likely won't be in a month, but one can give feedback by running extension from master branch or