Major changes include:

* Support for building against Proj 6.
* Support for Python 3.7 and 3.8.
* Support for Matplotlib 3.2 and 3.3.
* Support for labelling gridlines on all projections.
* Support for labelling gridlines within plot boundaries instead of
  edges.
* Support for labelling of contours.

and many others.

There are several bug fixes as well. See [the
milestone](https://github.com/SciTools/cartopy/milestone/25?closed=1)
for more information.