Readme updated

Author: os97673

README.md

@@ -23,6 +23,9 @@ To add another ruby version's source to this gem's directory:
     $ rake add_source VERSION=2.1.3 PATCHLEVEL=242
 
 (for pre-releases PATCHLEVEL should not be provided)
+add_source can use pre-downloaded .tgz (use TGZ_FILE_NAME to pass it)
+also it can extract patch level from version.h of downloaded sources, so
+PATCHLEVEL variable is optional.
 
 ##Credits