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/quickstarts/blob/master/.github/workflows/dapr-bot.yml#L33) in this repo.