Add README file to dev_tools subdirectory

[mhucka]

The dev_tools subdirectory contains a lot of scripts, most (perhaps all) of which are mentioned in other places in the documentation. Even if developers are not expected to cd into this directory and run things directly, it would still be useful to add a README to the directory itself to explain what it's about and how things are meant to be used.

[mhucka]

Closing this issue because it's subsumed by #6333.