doc: release notes for 2.18.0.0

Author: riddell-stan

doc/whats_new.rst

@@ -6,13 +6,13 @@
  What's New
 ============
 
-v2.18.0.0 (TBD)
+v2.18.0.0 (16. Aug 2018)
 ========================
 - Update Stan source to v2.18.0 (`release notes <https://github.com/stan-dev/stan/releases/tag/v2.18.0>`_)
 - Fit method ``to_dataframe`` organizes draws into a Pandas DataFrame. Only works if Pandas is installed. Thanks to Liam Brannigan and Ari Hartikainen.
 - Improved effective sample size calculation. Thanks to Aki Vehtari.
 - Implemented ``pystan.diagnostics``. Thanks to @jjramsey.
-- Several bug fixes.
+- Several minor bug fixes.
 
 v2.17.1.0 (16. Jan 2018)
 ========================