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
Adds tracking_url field to shipping methods

This feature adds the ability for tracking numbers to be clickable throughout
the system.

Without it, the customer needs to locate the tracking website themselves and
copy and paste the tracking number (except gmail users). It also effects admins
who are trying to investigate the history of a shipment, it would be helpful if
they could just click on the tracking and open up the tracking site.

Fixes #2644
[sample] Add missing spec folder
Revert "adds threading to chain of shipping_method.compute calls"

This reverts commit a582903d71e96ea2e0816be2e46587ce9ea8a40f.

This reverts PR #1976.

This fixes #2429.

Conflicts:
	core/app/models/spree/order.rb
Stop using @order in favor of order

order is passed as a local in every instance this partial is called

Fixes #2648
Merge branch 'master' of github.com:radar/spree