Fix release procedure document

Takayuki MATSUOKA · Takayuki MATSUOKA · commit 90b987ea41ef · 2017-04-13T01:53:41.000+09:00
diff --git a/releasing-procedure.md b/releasing-procedure.md
@@ -10,6 +10,8 @@
                 ...
 ```
 
+- Add tag
+
 - Push changes to the repository at github.
 
 - Create a `.unitypackage`
