chore: Add aws-session-token notation to README (#189)

I nearly gave up, believing it is not supported, until I read the action.yml. I think it is worth making this clear already in the README.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: holyjak

README.md

@@ -27,6 +27,7 @@ Add the following step to your workflow:
       with:
         aws-access-key-id: ${{ secrets.AWS_ACCESS_KEY_ID }}
         aws-secret-access-key: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
+        # aws-session-token: ${{ secrets.AWS_SESSION_TOKEN }} # if you have/need it
         aws-region: us-east-2
 ```