[Rosetta] add: rosetta docs

server/rosetta/README.md

@@ -0,0 +1,83 @@
+# Rosetta
+
+Package rosetta implements the rosetta API for the current cosmos sdk release series.
+
+The client satisfies [cosmos-rosetta-gateway](https://github.com/tendermint/cosmos-rosetta-gateway) `Client` interface implementation. 
+
+## Extension
+
+There are two ways in which you can customize and extend the implementation with your custom settings.
+
+### Message extension
+
+In order to make an `sdk.Msg` understandable by rosetta the only thing which is required is adding the methods to your message that satisfy the `rosetta.Msg` interface. 
+Examples on how to do so can be found in the staking types such as `MsgDelegate`, or in bank types such as `MsgSend`. 
+
+### Client interface override
+
+In case more customization is required, it's possible to embed the Client type and override the methods which require customizations.
+
+Example:
+```go
+package custom_client
+import (
+
+"context"
+"github.com/coinbase/rosetta-sdk-go/types"
+"github.com/cosmos/cosmos-sdk/server/rosetta"
+)
+
+// CustomClient embeds the standard cosmos client
+// which means that it implements the cosmos-rosetta-gateway Client
+// interface while at the same time allowing to customize certain methods
+type CustomClient struct {
+    *rosetta.Client
+}
+
+func (c *CustomClient) ConstructionPayload(_ context.Context, request *types.ConstructionPayloadsRequest) (resp *types.ConstructionPayloadsResponse, err error) {
+    // provide custom signature bytes
+    panic("implement me")
+}
+```
+
+### Error extension
+
+Since rosetta requires to provide 'returned' errors to network options. In order to declare a new rosetta error, we use the `errors` package in cosmos-rosetta-gateway.
+
+Example:
+
+```go
+package custom_errors
+import crgerrs "github.com/tendermint/cosmos-rosetta-gateway/errors"
+
+var customErrRetriable = true
+var CustomError = crgerrs.RegisterError(100, "custom message", customErrRetriable, "description")
+```
+
+Note: errors must be registered before cosmos-rosetta-gateway's `Server`.`Start` method is called. Otherwise the registration will be ignored. Errors with same code will be ignored too.
+
+## Integration in app.go
+
+To integrate rosetta as a command in your application, in app.go, in your root command simply use the `server.RosettaCommand` method. 
+
+Example:
+
+```go
+package app
+import (
+
+"github.com/cosmos/cosmos-sdk/server"
+"github.com/spf13/cobra"
+)
+
+func buildAppCommand(rootCmd *cobra.Command) {
+    // more app.go init stuff
+	// ...
+    // add rosetta command
+	rootCmd.AddCommand(server.RosettaCommand(encodingConfig.InterfaceRegistry, encodingConfig.Marshaler))
+}
+```
+
+A full implementation example can be found in `simapp` package.
+
+NOTE: when using a customized client, the command cannot be used as the constructors required **may** differ, so it's required to create a new one. We intend to provide a way to init a customized client without writing extra code in the future.