Add details explaining changes to from roles to Groups

[impactmass]

Things around the fact that:

The previous shop.defaultVisitorRoles are the roles now defined in the guest group.
Also, the previous shop.defaultCustomerRoles are the roles now defined in the default customer group.

So a previous reference to shop.defaultVisitorRoles should now use the group object (e.g Groups.find({slug: "guest"}).permissions).

The default list of those roles that we had on the Shop schema are present on server export of Reaction (i.e Reaction.default...)

Ref: reactioncommerce/reaction#2183

[machikoyasuda]

Closed w/ #311

[impactmass]

Hi @machikoyasuda .. #311 did not actually cover everything for this ticket. I could have combined then but didn't think about it. I can get this in soon too (should not be more than a few sentences).