Provide additional documentation on reading profile's custom fields

[aokomorowski]

Description

Hey, great job on this library, but to be honest I've found myself a little lost. I'm trying to retrieve custom fields on the user's profile. I've followed the discussion from #298 mentioned in the documentation, but I don't understand where I can take these fields from as userProfile.Fields.ToMap() appears to be empty (I've checked the API response by curl, it does return these fields).

I'd be extremely grateful for any tips or help.

(Optional) Slack's documentation

[aokomorowski]

Okay, I took a deeper look into the code and it seems like slack.Client.GetUsers() function uses users.list method which doesn't return custom fields.

https://api.slack.com/methods/users.list#profile