Include note about MAVEN_OPTS

CONTRIBUTING.md

@@ -39,6 +39,9 @@ can be build using the standard maven command:
 
 	$ mvn clean install
 
+> **NOTE:** You may need to increase the amount of memory available to Maven by setting
+> a `MAVEN_OPTS` environment variable with the value `-Xmx512m -XX:MaxPermSize=128m`
+
 If you are rebuilding often, you might also want to skip the tests until you are ready
 to submit a pull request:
 

README.md

@@ -232,6 +232,9 @@ latest and greatest, Spring Boot can be
 
 	$ mvn clean install
 
+> **NOTE:** You may need to increase the amount of memory available to maven by setting
+> a `MAVEN_OPTS` environment variable with the value `-Xmx512m -XX:MaxPermSize=128m`
+
 _Also see [CONTRIBUTING.md](CONTRIBUTING.md) if you wish to submit pull requests._
 
 ## Further Reading