docs: mention `devutils` in `README.rst`

[ycexiao]

Problem

It is from diffpy/diffpy.srreal#67 (comment).

devutils contains scripts we want to keep but are not user-facing. Since they are not in the src and there are no package API for them, it will be good to mention them in the README.rst.

Proposed solution

Mention devutils in the README like "Library of scripts used as part of the code development" .