Fix:Update readme

For the installaion for the stable release instead of installing v4 i.e., in beta right now.

Author: sahil-blulabs

README.md

@@ -63,6 +63,9 @@ $ git clone https://github.com/coreui/coreui-free-react-admin-template.git my-pr
 # go into app's directory
 $ cd my-project
 
+# checkout stable version
+$ git checkout master
+
 # install app's dependencies
 $ npm install
 ```