indentation clarification

docs/contribute/conventions.rst

@@ -89,6 +89,22 @@ This package follows all rules above, `check out the source
 in action.
 
 
+Indentation
+===========
+
+For Python files, we stick with the `PEP 8 recommondation
+<http://www.python.org/dev/peps/pep-0008/#indentation>`_: Use 4 spaces per
+indentation level.
+
+For ZCML and XML (GenericSetup) files, we recommend the `Zope Toolkit's coding
+style on ZCML <http://docs.zope.org/zopetoolkit/codingstyle/zcml-style.html>`_
+::
+
+  Indentation of 2 characters to show nesting, 4 characters to list attributes
+  on separate lines. This distinction makes it easier to see the difference
+  between attributes and nested elements.
+
+
 Quoting
 =======