Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -1 +1,43 @@
-Contributing
+## How to contribute
+- First, fork this repository
+- Second, clone repository in your develop machine
+- Finaly, writing code and push/PR to me(hahwul/gee)
+
+## Writing code
+I'm checking the quality of code through Codacy when PR/Merge/Push. If you want to consider code quality in advance, please check the link below (not perfect, but very helpful).
+
+https://goreportcard.com/report/github.com/hahwul/gee
+
+e.g: `https://goreportcard.com/report/github.com/{your github account}/gee`
+
+## Add new flag
+First, Add new flag in sturcture in `pkg/model/options.go`
+```go
+type Options struct {
+	Files         []string
+	Append        bool
+  // snip
+}
+```
+Second, Add flag in `main.go`
+```go
+// e.g
+appendOption := flag.Bool("append", false, "Append mode for files")
+...
+options := model.Options{
+		Files:         files,
+		Append:        *appendOption,
+    // snip 
+}
+```
+Finaly, Add logic
+```go
+if options.Append {
+  mode = os.O_APPEND | os.O_CREATE | os.O_WRONLY
+}
+```
+
+## Build
+```
+go build
+```