README: Document how to add PLATFORMSH_CLI_TOKEN env var to use the cli

[hanoii]

Unless there's some fancy way of making ddev platform login work, I'd say the best way is using a token:

https://docs.platform.sh/administration/cli/api-tokens.html

There are different ways of doing it, env var right now seems to work just fine:

You can have a config.platformsh.yaml file that you won't commit with:

web_environment:
  - PLATFORMSH_CLI_TOKEN=<YOUR_TOKEN>

Sorry I am not getting right into a PR, can't right now, but thought of at least sharing this.

[rfay]

It already does all that here... And it does it with a config.platformsh.yaml...

[hanoii]

ha, sorry, didn't see the install file.