docs: added instructions for yarn (#4706) [ci skip]

mpoehler · haoqunjiang · commit 48dd88affa95 · 2019-10-18T14:16:37.000+08:00
diff --git a/docs/migrating-from-v3/README.md b/docs/migrating-from-v3/README.md
@@ -8,6 +8,8 @@ First, install the latest Vue CLI globally:
 
 ```sh
 npm install -g @vue/cli
+# OR
+yarn global add @vue/cli
 ```
 
 ## Upgrade All Plugins at Once
