document dependencies

doc/general/requirements.rst

@@ -0,0 +1,16 @@
+
+
+Requirements and dependencies
+=============================
+
+The ``update.py`` script requires Python >= 2.7. We try to also
+support Python 3. If the script fails with Python 3, consider this
+as bug.
+
+The generated CMake infrastructure requires CMake >= 2.8.
+The generated ``setup.py`` runs with Python >= 2.7 (also Python 3).
+
+.. todo::
+
+   Figure out lower Python version bound for setup.py.
+   Figure out upper Python version bounds.

doc/index.rst

@@ -11,6 +11,7 @@ General
    :maxdepth: 2
 
    general/about.rst
+   general/requirements.rst
 
 
 For developers who use Autocmake