tock-register-interface: hint at the trait interfaces in README

Hint at the presence of the Readable, Writeable and ReadWriteable
traits in the crate's README, while still focusing on the included
types operating on memory-mapped registers. Furthermore, refer to the
proper trait documentation for more information.

Signed-off-by: Leon Schuermann <leon@is.currently.online>

Author: lschuermann

libraries/tock-register-interface/README.md

@@ -1,13 +1,14 @@
 # Tock Register Interface
 
-This crate provides an interface for defining and manipulating memory mapped
-registers and bitfields.
+This crate provides an interface and types for defining and
+manipulating registers and bitfields.
 
 ## Defining registers
 
 The crate provides three types for working with memory mapped registers:
 `ReadWrite`, `ReadOnly`, and `WriteOnly`, providing read-write, read-only, and
-write-only functionality, respectively.
+write-only functionality, respectively. These types implement the `Readable`,
+`Writeable` and `ReadWriteable` traits.
 
 Defining the registers is done with the `register_structs` macro, which expects
 for each register an offset, a field name, and a type. Registers must be
@@ -198,10 +199,12 @@ register_bitfields! [
 ## Register Interface Summary
 
 There are four types provided by the register interface: `ReadOnly`,
-`WriteOnly`, `ReadWrite`, and `Aliased`. They provide the following functions:
+`WriteOnly`, `ReadWrite`, and `Aliased`. They expose the following
+methods, through the implementations of the `Readable`, `Writeable`
+and `ReadWriteable` traits respectively:
 
 ```rust
-ReadOnly<T: IntLike, R: RegisterLongName = ()>
+ReadOnly<T: IntLike, R: RegisterLongName = ()>: Readable
 .get() -> T                                    // Get the raw register value
 .read(field: Field<T, R>) -> T                 // Read the value of the given field
 .read_as_enum<E>(field: Field<T, R>) -> Option<E> // Read value of the given field as a enum member
@@ -210,11 +213,11 @@ ReadOnly<T: IntLike, R: RegisterLongName = ()>
 .matches_all(value: FieldValue<T, R>) -> bool  // Check if all specified parts of a field match
 .extract() -> LocalRegisterCopy<T, R>          // Make local copy of register
 
-WriteOnly<T: IntLike, R: RegisterLongName = ()>
+WriteOnly<T: IntLike, R: RegisterLongName = ()>: Writeable
 .set(value: T)                                 // Set the raw register value
 .write(value: FieldValue<T, R>)                // Write the value of one or more fields,
                                                //  overwriting other fields to zero
-ReadWrite<T: IntLike, R: RegisterLongName = ()>
+ReadWrite<T: IntLike, R: RegisterLongName = ()>: Readable + Writeable + ReadWriteable
 .get() -> T                                    // Get the raw register value
 .set(value: T)                                 // Set the raw register value
 .read(field: Field<T, R>) -> T                 // Read the value of the given field
@@ -231,7 +234,7 @@ ReadWrite<T: IntLike, R: RegisterLongName = ()>
 .matches_all(value: FieldValue<T, R>) -> bool  // Check if all specified parts of a field match
 .extract() -> LocalRegisterCopy<T, R>          // Make local copy of register
 
-Aliased<T: IntLike, R: RegisterLongName = (), W: RegisterLongName = ()>
+Aliased<T: IntLike, R: RegisterLongName = (), W: RegisterLongName = ()>: Readable + Writeable
 .get() -> T                                    // Get the raw register value
 .set(value: T)                                 // Set the raw register value
 .read(field: Field<T, R>) -> T                 // Read the value of the given field
@@ -453,3 +456,14 @@ register_bitfields! [
     ]
 ]
 ```
+
+## Implementing custom register types
+
+The `Readable`, `Writeable` and `ReadWriteable` traits make it
+possible to implement custom register types, even outside of this
+crate. A particularly useful application area for this are CPU
+registers, such as ARM SPSRs or RISC-V CSRs. It is sufficient to
+implement the `Readable::get` and `Writeable::set` methods for the
+rest of the API to be automatically implemented by the crate-provided
+traits. For more in-depth documentation on how this works, refer to
+the `interfaces` module  documentation.