docs: Add contributing guide

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Contributing to Gmqtt
+We welcome contributions to Gmqtt of any kind including documentation, plugin, test, bug reports, issues, feature requests, typo fix, etc.  
+
+If you want to write some code, but don't know where to start or what you might want to do, take a look at the [Unplanned](https://github.com/DrmagicE/gmqtt/milestone/2) milestone.
+
+## Contributing Code
+Feel free submit a pull request. Any pull request must be related to one or more open issues.   
+If you are submitting a complex feature, it is recommended to open up a discussion or design proposal on issue track to get feedback before you start.
+
+### Code Style
+Gmqtt is a Go project, it is recommended to follow the [CodeReviewComments](https://github.com/golang/go/wiki/CodeReviewComments) guidelines.  
+When you’re ready to create a pull request, be sure to:
+* Have unit test for the new code.
+* Run [goimport](https://godoc.org/golang.org/x/tools/cmd/goimports).
+* Run go test -race ./... 
+* Build the project with race detection enable (go build -race .), and pass both V3 and V5 test cases (except test_flow_control2 [#68](https://github.com/eclipse/paho.mqtt.testing/issues/68)) in [paho.mqtt.testing](https://github.com/eclipse/paho.mqtt.testing/tree/master/interoperability)
+
+
+### Testing
+Testing is really important for building a robust application. Any new code or changes must come with unit test.
+
+#### Mocking
+Gmqtt uses [GoMock](https://github.com/golang/mock) to generate mock codes. The mock file must begin with the source file name and ends with `_mock.go`. For example, the following command will generate the mock file for `client.go`
+```bash
+mockgen -source=server/client.go -destination=/usr/local/gopath/src/github.com/DrmagicE/gmqtt/server/client_mock.go -package=server -self_package=github.com/DrmagicE/gmqtt/server
+```
+#### Assertion
+Please use [testify](https://github.com/stretchr/testify) for easy assertion.
+

README.md

@@ -120,6 +120,9 @@ See `/examples/hook` for details.
 ## How to write plugins
 [How to write plugins](https://github.com/DrmagicE/gmqtt/blob/master/plugin/README.md)
 
+# Contributing
+Contributions are always welcome, see [Contribution Guide](https://github.com/DrmagicE/gmqtt/blob/master/CONTRIBUTING.md) for a complete contributing guide.
+
 # Test
 ## Unit Test
 ```