Add documentation for KONAN_DATA_DIR variable

HACKING.md

@@ -20,6 +20,9 @@ There are several gradle flags one can use for Konan build.
 
         ./gradlew -Pprefix=external_codegen_box_cast run_external
 
+ ## Compiler environment variables
+
+* **KONAN_DATA_DIR** changes `.konan` local data directory location (`$HOME/.konan` by default). Works both with cli compiler and gradle plugin
 
  ## Testing
 

LIBRARIES.md

@@ -103,7 +103,7 @@ When given `-library foo` flag, the compiler searches the `foo`  library in the
 
     * All repositories specified with `-repo` flag.
 
-    * Libraries installed in the default repository (For now the default is  `~/.konan` ).
+    * Libraries installed in the default repository (For now the default is  `~/.konan`, however it could be changed by setting **KONAN_DATA_DIR** environment variable).
 
     * Libraries installed in `$installation/klib` directory.