[gtkmm-documentation] Created tag 3.20.0

[Murray Cumming <murrayc src gnome org>]

The unsigned tag '3.20.0' was created.

Tagger: Murray Cumming <murrayc murrayc com>
Date: Mon Mar 28 13:14:21 2016 +0200

    3.20.0

Changes since the last tag '3.19.3':

Kjell Ahlstedt (9):
      Use nullptr instead of 0
      cellrendererpopup example: Don't use deprecated methods
      Update the Internationalization chapter
      Custom widget example: Avoid some warnings
      builder/derived example: Add constructor with additional parameter
      Builder chapter: Describe additional parameters to get_widget_derived()
      Add "String Parameter Processing" section
      Custom widget and ListBox examples: Update use of CSS files
      ProgressBar example: Fix the type of a data member

Murray Cumming (2):
      Add parentheses to silence clang-tidy.
      3.20