Update the "how to contribute" docs with py3k info.

docs/contribute.rst

@@ -58,6 +58,21 @@ To do so, you can use the following two commands::
         samples/content/
     $ LC_ALL="C" pelican -o tests/output/basic/ samples/content/
 
+testing for python3
+-------------------
+
+On Python 3, if you have installed the Py3k compatible versions of the
+plugins manual testing with ``unit2 discover`` is also straightforward.
+
+However, you must tell tox to use those Py3k libraries. If you forget this,
+tox will pull the regular packages from PyPi and the tests will fail.
+
+Tell tox about the local packages thusly: enter the source directory of
+smartypants and run tox there. Do this again for typogrify and webassets.
+Smartypants and typogrify do not have real tests, and webassets will fail
+noisily, but as a result we get these libraries neatly packaged in tox's
+distshare directory. And this we need to run tox for Pelican.
+
 Coding standards
 ================
 
@@ -67,3 +82,44 @@ eased via the `pep8 <http://pypi.python.org/pypi/pep8>`_ or `flake8
 <http://pypi.python.org/pypi/flake8/>`_ tools, the latter of which in
 particular will give you some useful hints about ways in which the
 code/formatting can be improved.
+
+Python3 support
+===============
+
+Here are some tips that may be useful when doing some code for both python2 and
+python3 at the same time:
+
+- Assume, every string and literal is unicode (import unicode_literals):
+ 
+  - Do not use prefix ``u'``.
+  - Do not encode/decode strings in the middle of sth. Follow the code to the
+    source (or target) of a string and encode/decode at the first/last possible
+    point.
+  - In other words, write your functions to expect and to return unicode.
+  - Encode/decode strings if e.g. the source is a Python function that is known
+    to handle this badly, e.g. strftime() in Python 2.
+
+- Use new syntax: print function, "except ... *as* e" (not comma) etc.
+- Refactor method calls like ``dict.iteritems()``, ``xrange()`` etc. in a way
+  that runs without code change in both Python versions.
+- Do not use magic method ``__unicode()__`` in new classes. Use only ``__str()__``
+  and decorate the class with ``@python_2_unicode_compatible``.
+- Do not start int literals with a zero. This is a syntax error in Py3k.
+- Unfortunately I did not find an octal notation that is valid in both
+  Pythons. Use decimal instead.
+- use six, e.g.:
+
+  - ``isinstance(.., basestring) -> isinstance(.., six.string_types)``
+  - ``isinstance(.., unicode) -> isinstance(.., six.text_type)``
+
+- ``setlocale()`` in Python 2 bails when we give the locale name as unicode,
+  and since we are using ``from __future__ import unicode_literals``, we do
+  that everywhere!  As a workaround, I enclosed the localename with ``str()``;
+  in Python 2 this casts the name to a byte string, in Python 3 this should do
+  nothing, because the locale name already had been unicode.
+
+- Kept range() almost everywhere as-is (2to3 suggests list(range())), just
+  changed it where I felt necessary.
+
+- Changed xrange() back to range(), so it is valid in both Python versions.
+