Add link to unity-2019.4-mbe branch

Author: joncham

README.md

@@ -18,6 +18,9 @@ Branches for released Unity versions are of the form unity-\<version\>\[-mbe\][-
 #### 2020.1
 * [unity-2020.1-mbe](https://github.com/Unity-Technologies/mono/tree/unity-2020.1-mbe)
 
+#### 2019.4
+* [unity-2019.4-mbe](https://github.com/Unity-Technologies/mono/tree/unity-2019.4-mbe)
+
 #### 2019.3
 * [unity-2019.3-mbe](https://github.com/Unity-Technologies/mono/tree/unity-2019.3-mbe)