docs: add manual for adding rpc types (#85)

docs/how_to_add_new_interface.md

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+# Howto: Add New RPC Interface
+
+This document is lastly edited by Tao Wu ([email protected]), 2021/4/28.
+
+This document illustrates the steps of adding a new RPC interface to the client.
+
+## Steps
+
+1. Add the RPC's thrift definitions to `idl/`.
+
+You can write definitions to a new file, for example, foo.thrift.
+Or you can just write them to existing files. It depends on which module that the new interface belongs to.
+Currently, `idl/admin.thrift` contains all MetaServer administration RPCs,
+`idl/radmin.thrift` contains the RPCs all ReplicaServer administration RPCs.
+
+2. Prepare the thrift compiler.
+
+pegasus-go-client uses thrift-compiler with 0.13.0, which is the version you can directly install via `apt-get` on Ubuntu 20.04.
+If the compiler is missing on your OS,
+you can follow [the building manual of thrift](https://thrift.apache.org/docs/BuildingFromSource) and build it yourself.
+
+3. Compile thrift files to generate go code.
+
+```sh
+thrift -I idl -out idl --gen go:thrift_import='github.com/pegasus-kv/thrift/lib/go/thrift',package_prefix='github.com/XiaoMi/pegasus-go-client/idl/' idl/admin.thrift
+gofmt -w **/*.go
+goimports -w */**.go
+```
+
+4. Add functions to `session/meta_session.go` or `session/replica_session.go` for the new RPC.
+
+In this step no hand-writing code is required. You merely need to append a tuple of "TaskCode,RequestType,ResponseType"
+to `generator/admin.csv` or `generator/radmin.csv`.
+Then run `make` which automatically generates code according to the template.
+
+5. Register the callback for the new RPC.
+
+This step is trivial, and the code can be auto-generated in the future. 
+For now, you should go `session/codec.go` and add a new response callback to `nameToResultMap`.
+The example code:
+
+```go
+"RPC_RRDB_RRDB_INCR_ACK": func() RpcResponseResult {
+    return &rrdb.RrdbIncrResult{
+        Success: rrdb.NewIncrResponse(),
+    }
+},
+```