Put DifferentialEquation at the top of release

Author: Dpananos

RELEASE-NOTES.md

@@ -3,13 +3,12 @@
 ## PyMC3 3.8 (on deck)
 
 ### New features
-
+- Add capabilities to do inference on parameters in a differential equation with `DifferentialEquation`.
 - Distinguish between `Data` and `Deterministic` variables when graphing models with graphviz. PR [#3491](https://github.com/pymc-devs/pymc3/pull/3491).
 - Sequential Monte Carlo - Approximate Bayesian Computation step method is now available. The implementation is in an experimental stage and will be further improved.
 - Added `Matern12` covariance function for Gaussian processes. This is the Matern kernel with nu=1/2.
 - Progressbar reports number of divergences in real time, when available [#3547](https://github.com/pymc-devs/pymc3/pull/3547).
 - Sampling from variational approximation now allows for alternative trace backends [#3550].
-- Add capabilities to do inference on parameters in a differential equation with `DifferentialEquation`.
 
 ### Maintenance
 - Moved math operations out of `Rice`, `TruncatedNormal`, `Triangular` and `ZeroInflatedNegativeBinomial` `random` methods. Math operations on values returned by `draw_values` might not broadcast well, and all the `size` aware broadcasting is left to `generate_samples`. Fixes [#3481](https://github.com/pymc-devs/pymc3/issues/3481) and [#3508](https://github.com/pymc-devs/pymc3/issues/3508)