Update changelog to include grid change

Author: simonrw

CHANGELOG

@@ -8,6 +8,7 @@
 - Introduce categorical representation, views and axes.
 - Add ability to set dimensions of plot (PR #8)
 - Added ability to display a histogram as densities
+- Add ability to display grids ([PR #23](https://github.com/milliams/plotlib/pull/23))
 
 ### Changed
 - Change `create_axes`, `save`, `to_svg` and `to_text` to return `Result` indicating an error.