Make a note about the internal tools/ dir and what each utility does

tools/README.md

@@ -0,0 +1,8 @@
+## Rails dev tools
+
+This is a collection of utilities used for Rails internal development.
+They aren't used by Rails apps directly.
+
+  * `console` drops you in irb and loads this local Rails repos
+  * `line_statistics` gives lines-of-code counts for our codebase
+  * `profile` profiles `Kernel#require` to help reduce startup times