Improve documentation of the Custom Queries feature

The following thread from #326 (closed) should be addressed:

• @viktomas commented on a thread:

• It's unclear how custom queries are set, especially that if you set them in your settings, you completely override the existing defaults.

  • I'd suggest solving this by creating an issue to separate the custom query configuration into a separate doc (now it takes ~20% of the README and users can't see the important points) and in the README.md only summarize that you can fully customize the queries that you see in the "issues and merge requests" sidebar.

Improvements should be made to clearly explain the difference between the "default queries" and "custom queries", if there even is meant to be a difference at all. If not, it should be clarified that the "default queries" can easily be removed/modified by simply changing the "custom queries" setting.