added Mac setup instructions (#2820)

* added Mac setup instructions

java/demo/README

@@ -1,6 +0,0 @@
-Prerequisite:
-  Install the Appengine SDK.
-  $ sudo apt-get install google-cloud-sdk-app-engine-java
-
-How to regenreate the WAR, deploy and run in appengine?
-  $ mvn appengine:run

java/demo/README.md

@@ -0,0 +1,15 @@
+## Prerequisite:
+
+### Linux
+#### Install the Appengine SDK.
+`$ sudo apt-get install google-cloud-sdk-app-engine-java`
+
+### Mac
+#### Install the Google Cloud SDK.
+`$ brew install google-cloud-sdk`
+
+#### Use GCloud CLI to install the Java Appengine
+`$ gcloud components install app-engine-java`
+
+## How to regenerate the WAR, deploy and run in appengine?
+`$ mvn appengine:run`