Mention urwid as dependency

README.md

@@ -32,6 +32,8 @@ Simply install with:
                         (not supported for Python 3.2).
 * [prompt-toolkit][6] : For topydo's _prompt_ mode, which offers a shell-like
                         interface with auto-completion.
+* [urwid][12]         : For topydo's _columns_ mode, a TUI with columns for
+*                       your todo items.
 * [arrow][8]          : Used to turn dates into a human readable version.
 * [backports.shutil_get_terminal_size][9] : Used to determine your terminal
                                             window size. This function was
@@ -60,3 +62,4 @@ Demo
 [9]: https://github.com/chrippa/backports.shutil_get_terminal_size
 [10]: https://dateutil.readthedocs.org/
 [11]: https://github.com/testing-cabal/mock
+[12]: https://github.com/urwid/urwid