Document That Users Can Self-Assign Issues

Please link to the [daprbot documentation](https://docs.dapr.io/contributing/daprbot/) inside the contributing.md file, so that users know what Github commands they can run in this repo. Based on the dapr-bot.yml they can run the [/assign to self assign issues](https://github.com/dapr/cli/blob/master/.github/workflows/dapr-bot.yml) in this repo.