Update README.rst (#3291)

Add new software link for exoplanet
cc @dfm

Author: junpenglao

README.rst

@@ -143,6 +143,7 @@ License
 Software using PyMC3
 ====================
 
+- `Exoplanet <https://github.com/dfm/exoplanet>`__: a toolkit for modeling of transit and/or radial velocity observations of exoplanets and other astronomical time series.
 - `Bambi <https://github.com/bambinos/bambi>`__: BAyesian Model-Building Interface (BAMBI) in Python.
 - `pymc3_models <https://github.com/parsing-science/pymc3_models>`__: Custom PyMC3 models built on top of the scikit-learn API.
 - `PMProphet <https://github.com/luke14free/pm-prophet>`__: PyMC3 port of Facebook's Prophet model for timeseries modeling