docs: vcs_setup: add note about Git HTTP POST buffer

As suggested by https://bitbucket.org/conservancy/kallithea/issues/229/bad-file-descriptor#comment-30073565

docs/administrator_guide/vcs_setup.rst

@@ -1,52 +1,61 b''
 .. _vcs_setup:
 
 =============================
 Version control systems setup
 =============================
 
 Kallithea supports Git and Mercurial repositories out-of-the-box.
 For Git, you do need the ``git`` command line client installed on the server.
 
 You can always disable Git or Mercurial support by editing the
 file ``kallithea/__init__.py`` and commenting out the backend. For example, to
 disable Git but keep Mercurial enabled:
 
 .. code-block:: python
 
    BACKENDS = {
        'hg': 'Mercurial repository',
        #'git': 'Git repository',
    }
 
 
 Git-specific setup
 ------------------
 
 
 Web server with chunked encoding
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Large Git pushes require an HTTP server with support for
 chunked encoding for POST. The Python web servers waitress_ and
 gunicorn_ (Linux only) can be used. By default, Kallithea uses
 waitress_ for `gearbox serve` instead of the built-in `paste` WSGI
 server.
 
 The web server used by gearbox is controlled in the .ini file::
 
     use = egg:waitress#main
 
 or::
 
     use = egg:gunicorn#main
 
 Also make sure to comment out the following options::
 
     threadpool_workers =
     threadpool_max_requests =
     use_threadpool =
 
+Increasing Git HTTP POST buffer size
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If Git pushes fail with HTTP error code 411 (Length Required), you may need to
+increase the Git HTTP POST buffer. Run the following command as the user that
+runs Kallithea to set a global Git variable to this effect::
+
+    git config --global http.postBuffer 524288000
+
 
 .. _waitress: http://pypi.python.org/pypi/waitress
 .. _gunicorn: http://pypi.python.org/pypi/gunicorn
 .. _subrepositories: http://mercurial.aragost.com/kick-start/en/subrepositories/