Document RPC framework (#501)

Fixes #500

src/rpc/README.md

@@ -0,0 +1,29 @@
+# Clio RPC subsystem
+## Background
+The RPC subsystem is where the common framework for handling incoming JSON requests is implemented.
+Currently the NextGen RPC framework is a work in progress and the handlers are not yet implemented using the new common framework classes.
+
+## Integration plan
+- Implement base framework - **done**
+- Migrate handlers one by one, making them injectable, adding unit-tests - **in progress**
+- Integrate all new handlers into clio in one go
+- Cover the rest with unit-tests
+- Release first time with new subsystem active
+
+## Components
+See `common` subfolder.
+
+- **AnyHandler**: The type-erased wrapper that allows for storing different handlers in one map/vector.
+- **RpcSpec/FieldSpec**: The RPC specification classes, used to specify how incoming JSON is to be validated before it's parsed and passed on to individual handler implementations.
+- **Validators**: A bunch of supported validators that can be specified as requirements for each **`FieldSpec`** to make up the final **`RpcSpec`** of any given RPC handler.
+
+## Implementing a (NextGen) handler
+See `unittests/rpc` for exmaples.
+
+Handlers need to fulfil the requirements specified by the **`Handler`** concept (see `rpc/common/Concepts.h`):
+- Expose types: 
+	* `Input` - The POD struct which acts as input for the handler
+	* `Output` - The POD struct which acts as output of a valid handler invocation
+- Have a `spec()` member function returning a const reference to an **`RpcSpec`** describing the JSON input.
+- Have a `process(Input)` member function that operates on `Input` POD and returns `HandlerReturnType<Output>`
+- Implement `value_from` and `value_to` support using `tag_invoke` as per `boost::json` documentation for these functions.