Update README.md

Author: Spencer Cai

README.md

@@ -1,3 +1,14 @@
 # git-branch-behind-main
 
-Check whether the current branch is behind main
+Check whether the current branch is behind main.
+
+Just add the following step to your workflow:
+
+```yaml
+- name: git-branch-behind-main
+  uses: spencercjh/git-branch-behind-main@v1.0.0
+```
+
+## TBD
+
+1. Now only the Chinese error message is available, which should be configurable