Add to documentation - instantiate multiple instances

The method suggested in #108 for instantiating multiple instances of tabby with the same selector might be a useful addition to the documentation. It was exactly what I was looking for, and discovered it in the closed issues.

Read closed issue #108 first to identify the method for instantiating multiple Tabby instances with the same selector. Locate the documentation section covering initialization, add the method and its usage, and confirm that readers can follow it to instantiate multiple instances.

Written by the indexing model from the issue text.