Add version update info for v0.13 in CHANGES.rst
- Planned for July 15, 2019
+The recommended conda environment for Gammapy v0.13 contains some version updates:
+Python 3.7, ipython 7.5, Scipy 1.3, Matplotlib 3.1, Astropy 3.1, healpy 1.12.
+These updates should be backward compatible, your scripts and notebooks should
+run and give the same outputs.
.. _gammapy_0p12_release:
