Update README-InstallingIntelliJ.md

Author: Git-Leon

README-InstallingIntelliJ.md

@@ -5,4 +5,6 @@
     * `/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"`
 2. After HomeBrew has been installed, copy and paste the following command in the terminal
     * `brew cask install intellij-idea`
-    
+3. Upon launching IntelliJ, ensure the [Apache Maven](https://maven.apache.org/) plugin is enabled.
+   * From the menu bar found at the top of application, select `Plugins`, and ensure `Maven` is checked.
+