update readme

README.md

@@ -2,11 +2,18 @@
 
 Go port of [degit](https://github.com/rich-harris/degit).
 
+Usage
+
+```bash
+degit user/repo#ref output-dir
+```
+
+This downloads the github repository `https://github.com/user/repo" at "ref", ref could be a branch, a tag or commit hash. If ref is empty, the main branch will be used. You can specify subdirectories and use Gitlab and Bitbucket repositories as well. degit also maintains a cache to save downloads and keep refs updated.
+
+## Installation
+
 ```bash
 brew tap qiushiyan/degit https://github.com/qiushiyan/degit
 brew install qiushiyan/degit/degit
 
-degit user/repo#ref output-dir
 ```
-
-Downloads the github repository locally. You can specify subdirectories and use Gitlab and Bitbucket repositories as well. If the commit hash does not change, degit uses the cached version to save downloading again.