Add documentation to show usage with multiple variables

[ctsstc]

I noticed in a project I'm working on they've included the uses twice in the github action, but upon looking at source code for this repository it seems that you can utilize a comma to delimit multiple secrets.

It would be nice to give an example in the readme for this usage.

I also imagine there cannot be a space included. It might be helpful to trim the secrets first unless the action core package is secretly doing this for us.