Add a script changing file extensions

README.md

@@ -57,4 +57,10 @@ script should work on Mac OS with preinstalled GNU tools and case-sensitive file
 system and Linux. The resulting GCC is to be found in
 `./tmp/rootfs/bin/riscv32-unknown-linux-gnu-*`.
 
+## rename.rb
+
+Changes the extension of all files matching a pattern in the given directory.
+Call the `rename.rb` without arguments to see the usage information and an
+example.
+
 ## tea-cleaner