Document device actions sample support

[proppy]

This came up in #356 that device actions support in the sample is not very discoverable.

We should consider:

• [ ] move the actions.json in the pushtotalk directory
• [ ] add instruction to deploy the actions package in the README.md
• [ ] document the step to try both built-in and custom device actions

[Shulyaka]

Hi! How do I register the timer trait? It's not in the list of the available traits: https://developers.google.com/assistant/sdk/guides/service/python/extend/register-device-traits. Would also love to see traits for alarm and news...