doc: protocol units.

Author: l-monninger

protocol-units/README.md

@@ -2,4 +2,7 @@
 `protocol-units` are used to collect runnable and composable units for interacting with the `ffs` protocol. We store contracts within protocol units, for example. 
 
 ## Contents
-- **[`mcr`](./mcr/)**: the `MCR` implementation of `ffs` provided prior to the specification of the `fast-confirmations` and `post-confirmations` protocols. 
+- **[`mcr`](./mcr/)**: the `MCR` implementation of `ffs` provided prior to the specification of the `fast-confirmations` and `post-confirmations` protocols. It is maintained for legacy purposes.
+- **[`post-confirmations`](./post-confirmations/)**: the `post-confirmations` sub-protocol implementation for `ffs`. 
+- **[`fast-confirmations`](./fast-confirmations/)**: the `fast-confirmations` sub-protocol implementation for `ffs`.
+- **[`ffs`](./ffs/)**: the full `ffs` protocol implementation.