Adding documentation for translators

We have changed to xgettext creating first version of documentation.
This can be converted into a nice pdf by rst2pdf.

docs/translation.rst

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+***********
+Translation
+***********
+
+DAViCAL uses the standard translation infrastructure of many projects. 
+It is based on ``gettext`` (http://www.gnu.org/software/gettext/).
+
+Translators
+===========
+
+Translating DAViCal
+-------------------
+
+All translation of DAViCal is done by the Transifex tool (http://transifex.org/).
+We are using the Transifex service (http://www.transifex.net/) to translate the software.
+
+To translate DAViCal go to http://www.transifex.net/projects/p/davical.
+You can register an account there and then translate DAViCal in the webbrowser to your language.
+Furthermore you can improve existing translations and join the team of your language.
+
+Developers
+==========
+
+Adding translatable string to the PHP-Source
+--------------------------------------------
+
+DAViCal currently does not use ``_()`` but has two functions to do the translation.
+These functions are named ``i18n()`` and ``translate()``.
+
+With ``translate()`` a message can be directly translated.
+
+.. code-block:: php
+
+   <?php
+   print translate('TEST to be displayed in different languages');
+
+In case a variable is passed all candidates can be marked by ``i18n()``.
+
+.. code-block:: php
+
+   <?php
+   $message_to_be_localized = i18n('TEST to be displayed in different languages');
+   print translate($message_to_be_localized);
+
+Adding context as a help for the translators
+--------------------------------------------
+
+There are cases in which a translation might be ambigous for the translators at these places hints may be inserted.
+
+Consider the following code fragment.
+
+.. code-block:: php
+
+   <?php
+   # Translators: short for 'Number'
+   i18n("No.");
+   
+   # Translators: not 'Yes'
+   i18n("No");
+
+With the ``Translators:`` keyword in a comment directly in front of the line with the string to be translated such hints can be marked.
+
+Generating the .pot file from source
+------------------------------------
+
+To generate a .pot file for translators on has to use the following command:
+
+.. code-block:: bash
+
+  $ xgettext -f po/pofilelist.txt --keyword=i18n --keyword=translate \
+   --add-comments=Translators
+  $ sed -i 's.^"Content-Type: text/plain; charset=CHARSET\\n".\
+  "Content-Type: text/plain; charset=UTF-8\\n".' messages.po
+  $ mv messages.po messages.pot
+
+This will generate a file named ``messages.pot``.
+