Add explanations of components to README.md

Not entirely sure about the backend/frontend distinction since it just happened, so hopefully I got this right.

Fixes #2653
Minor grammar correction in Core Gemspec

'Barebones' without a space functions as an adjective. In this case, we're referring to the gem, so using 'bones' as a noun is correct.

Fixes #2657
Fix for failing authorization when modifying the taxonomy tree

Issue: when modifying the taxonomy tree (create, update, destroy), authorization
is required but fails when Spree::Api::Config[:requires_authentication] is false

Fixes #2659
fix: update_positions is not a rest action so it should not call load_resource

Fixes #2627
Fix percent per item calculator to work when no rules restricting products.

Fixes #2660
Fix issue with count_on_hand attribute in sample loading (#2631)
Update activemerchant to requiring at least 1.31.x

Fixes #2661