chore: and cmi long desc

Author: cadenmyers13

README.rst

@@ -37,7 +37,9 @@
 
 Complex modeling infrastructure: a modulare framework for multi-modal modeling of scientific data.
 
-* LONGER DESCRIPTION HERE
+The software provides functionality for storage and manipulation of structure data and calculation of structure-based quantities, such as PDF, SAS, bond valence sums, atom overlaps, bond lengths, and coordinations. Most importantly the DiffPy-CMI package contains a fitting framework for combining multiple experimental inputs in a single optimization problem.
+
+This is an early release of code that is under intense development, with support for installation on Unix, Linux, and Macintosh machines. The scope and documentation of the project will evolve rapidly, but we want to make the code available at the earliest possible date. Please make use of the software and provide feedback and suggestions for improvement, but please be patient and check back frequently for updates.
 
 For more information about the diffpy.cmi library, please consult our `online documentation <https://diffpy.github.io/diffpy.cmi>`_.