refine help doc and add readme

README.md

@@ -0,0 +1,27 @@
+# git-chart
+
+`gitchart` is a command line tool to show git commits distribution in the last several months.
+
+![](./example.png)
+
+
+
+## Install
+
+```shell
+go get github.com/chenlujjj/gitchart
+```
+
+## Usage
+
+```
+$ gitchart -h
+
+Usage of gitchart:
+  -month int
+        since how many months ago (default 6)
+  -self
+        only count commits by myself. This flag would override 'username' flag
+  -username string
+        count commits by the 'username' author. If not set, would count all authors' commits
+```

main.go

@@ -17,8 +17,8 @@ const cellWidth = 3        // 每个格子的宽度
 
 func main() {
 	months := flag.Int("month", 6, "since how many months ago")
-	username := flag.String("username", "", "check commits by the author. If not set, would check all authors' commits")
-	self := flag.Bool("self", false, "only check commits by myself. This flag would override `username` flag")
+	username := flag.String("username", "", "count commits by the 'username' author. If not set, would count all authors' commits")
+	self := flag.Bool("self", false, "only count commits by myself. This flag would override 'username' flag")
 
 	flag.Parse()
 	repo, err := os.Getwd()