Describe some dependancy changes in API break docs

Author: nyalldawson

doc/api_break.dox

@@ -21,6 +21,31 @@ This page tries to maintain a list with incompatible changes that happened in pr
 QGIS 3.0        {#qgis_api_break_3_0}
 ========
 
+Version 3.0 brings changes to many underlying dependancies which QGIS is built upon. Any existing PyQGIS code will
+need to be updated to address the changes made within these libraries.
+
+Python 3.0
+----------
+
+QGIS 3.0 introduces a move to Python 3. This version brings many changes to both the Python language and individual Python
+libraries. A good place to start learning about the changes involved, and how to port your code to Python 3, is available
+in the official Python documentation: [Porting Python 2 Code to Python 3](https://docs.python.org/3/howto/pyporting.html).
+
+Qt 5
+----
+
+QGIS 3.0 is based off version 5 of the underlying Qt libraries. Many changes and API breaks were introduced in Qt5. While
+it is c++ specific, a good place to read about the major changes introduced in Qt5 is at the Qt docs:
+[C++ API changes](http://doc.qt.io/qt-5/sourcebreaks.html)
+
+
+PyQt 5
+------
+
+Together with the Python and Qt version changes, the PyQt libraries which expose Qt classes to Python have also had their
+version bumped to PyQt 5. The changes introduced in PyQt 5 and the steps required to upgrade existing code are summarised at:
+[Differences Between PyQt4 and PyQt5](http://pyqt.sourceforge.net/Docs/PyQt5/pyqt4_differences.html)
+
 Moved Classes        {#qgis_api_break_3_0_moved_classes}
 -------------