doc: Add markdown document for command detection framework (#321)

Add document for command detction framework:

- naming rules
- signature rules
- embedding struct

Signed-off-by: Congqi Xia <[email protected]>

docs/bw_framework.md

@@ -0,0 +1,42 @@
+# Birdwatcher command framework
+
+Using `AddCommand` from `cobra.Command` for each command is annoying. Birdwatcher implemented its own command definition and detection framework. It could be done by defining two entities for one command.
+
+## Auto detection
+
+All *public* method of `states.State` will be iterated to check whether it is a "Command Function" or not. One method shall be a "Command Function", which means it will be auto registered as a command if:
+
+- The method name is in format of "XXXXXCommand", which must be ended with "Command", case sensitive.
+- The method must have two input parameters
+    - First parameter shall be `context.Context`
+    - Second shall be a pointer of a struct implementing `framework.CmdParam`
+- The return type list could be:
+    - Just one value, which is in type of `error` standing for whether the execution is successful or not.
+    - Two values, the second is in type of `error`, and the first one is in type which implements `framework.ResultSet`
+
+Here is one example
+
+``` go
+// states/etcd_connect.go
+
+type ConnectParams struct {
+	framework.ParamBase `use:"connect" desc:"Connect to etcd"`
+    EtcdAddr            string `name:"etcd" default:"127.0.0.1:2379" desc:"the etcd endpoint to connect"`
+    // omit other detail definition
+}
+
+func (s *disconnectState) ConnectCommand(ctx context.Context, cp *ConnectParams) error {
+    // ...
+    return nil
+}
+```
+
+`ConnectCommand` will be registerer using the golang tag of `framework.ParamBase`, which saying the command shall use `connect`. This is the most common command used in birdwatcher.
+
+And the `etcd` param is alos defined via tagging feature. Framework auto detects the type from the type `string` and set the param name with `name` tag, the default value from `default` tag and the description text from `desc` tag.
+
+## Embed component
+
+If the commands are many, if could be hard to maintain for just one state with many methods. Actually in current implementation, etcd show commands are often defined under `state/etcd` package.
+
+So birdwatcer framework also provided the ability to read anonymous embedded component method to scan command candiates as well.