Add a note about JDK 6 path required to build this project

Author: ilya-g

README.md

@@ -159,6 +159,9 @@ implementation 'org.jetbrains.kotlinx:kotlinx-collections-immutable:0.2'
 
 ## Building from source
 
+> :information_source: To build this project you will need to have a path to JDK 6 specified with the `JDK_6` environment variable or gradle property.
+> For the local development purposes any JDK newer than that can be used instead.
+
 You can build and install artifacts to maven local with:
 
     gradlew build install