Add registry readme (#351)

README.md

@@ -17,6 +17,8 @@ This package is feature complete, but it is relatively new and might still conta
 
 Please refer to https://godoc.org/github.com/centrifuge/go-substrate-rpc-client
 
+### Usage test examples of Dynamic Parsing of events & extrinsics
+[Registry docs](registry/REGISTRY.md)
 ## Contributing
 
 1. Install dependencies by running `make`

registry/REGISTRY.md

@@ -0,0 +1,22 @@
+# GSRPC Registry
+The GSRPC Registry can parse target metadata information into an in-memory registry of complex structures. 
+
+By leveraging the on-chain metadata, GSRPC is more robust to changes on types, allowing clients to only keep updated the types that are relevant to their business operation.
+
+This registry can be used afterwards to decode data read from live chains (events & extrinsics).
+
+## Usage
+Since docs get outdated fairly quick, here are links to tests that will always be up-to-date.
+### Populate Call, Error & Events Registries
+[Browse me](registry_test.go)
+
+### Event retriever
+[TestLive_EventRetriever_GetEvents](retriever/event_retriever_live_test.go)
+### Extrinsic retriever
+Since chain runtimes can be customized, modifying core types such as Accounts, Signature payloads or Payment payloads, the code supports a customizable way of passing those custom types to the extrinsic retriever.
+
+On the other hand, since a great majority of chains do not need to change these types, the tool provides a default for the most common used ones.
+#### Using Chain Defaults
+[TestExtrinsicRetriever_NewDefault](retriever/extrinsic_retriever_test.go#L179)
+#### Using Custom core types
+[TestLive_ExtrinsicRetriever_GetExtrinsics](retriever/extrinsic_retriever_live_test.go)