Added some tips to the contributor docs, namely about removing trailing...

    * Remove ``import`` statements that are no longer used when you change code.

      The most common tools for this task are `pyflakes`_ and `pylint`_.

    * Systematically remove all trailing whitespaces from your code as those

      add unnecessary bytes, add visual clutter to the patches and can also

      occasionally cause unnecessary merge conflicts. Some IDE's can be

      configured to automatically remove them and most VCS tools can be set to

      highlight them in diff outputs.

    * Please don't put your name in the code you contribute. Our policy is to

      keep contributors' names in the ``AUTHORS`` file distributed with Django

      -- not scattered throughout the codebase itself. Feel free to include a

    * The code required to fix a problem or add a feature is an essential part

      of a patch, but it is not the only part. A good patch should also

      include a regression test to validate the behavior that has been fixed,

      to prevent the problem from arising again.

      include a regression test to validate the behavior that has been fixed

      and to prevent the problem from arising again. Also, if some tickets are

      relevant to the code that you've written, mention the ticket numbers in

      some comments in the test so that one can easily trace back the relevant

      discussions after your patch gets committed and the tickets get closed.

    * If the code associated with a patch adds a new feature, or modifies

      behavior of an existing feature, the patch should also contain