Add scopes list to README.md

[antonio-ramadas]

First of all, great job. This is really useful! I just have this minor suggestion of improvement.

When setting up the app, there could be more screenshots showing the addition of scopes. The list is quite extensive and is not easily shown to the user (I had to dive in into the source code). Adding at least one screenshot mentioning the necessary steps would be useful :)

[yannickglt]

Hi,
Thank you for using the Slack workflow for Alfred!
I added some steps and screenshots to the readme in b75eb61, hopefully it will be easier to understand.