Fix git commit typo (#1347)

* Refine the git commit steps description.

README.md

@@ -84,7 +84,7 @@ So that you can start straight away.
 
 - Fork it!
 - Create your feature branch: `git checkout -b my-new-feature`
-- Commit your changes: `git commit -am 'Add some feature'`
+- Commit your changes: `git add . && git commit -m 'Add some feature'`
 - Push to the branch: `git push origin my-new-feature`
 - Submit a pull request