Add troubleshooting questions regarding updating

docs/source/installation/troubleshooting-common.rst

@@ -19,6 +19,65 @@ I’m using tmux/screen and Powerline is colorless
   to ``"tmux"`` or ``"screen"``. Note that it is known to work perfectly in 
   screen, but in tmux it may produce ugly spaces.
 
+After an update something stopped working
+-----------------------------------------
+
+Assuming powerline was working before update and stopped only after there are 
+two possible explanations:
+
+* You have more then one powerline installation (e.g. ``pip`` and ``Vundle`` 
+  installations) and you have updated only one.
+* Update brought some bug to powerline.
+
+In the second case you, of course, should report the bug to `powerline bug 
+tracker <https://github.com/Lokaltog/powerline>`_.  In the first you should make 
+sure you either have only one powerline installation or you update all of them 
+simultaneously (beware that in the second case you are not supported). To 
+diagnose this problem you may do the following:
+
+#) If this problem is observed within the shell make sure that
+
+   .. code-block:: shell
+
+      python -c 'import powerline; print (powerline.__file__)'
+
+   which should report something like 
+   :file:`/usr/lib64/python2.7/site-packages/powerline/__init__.pyc` (if 
+   powerline is installed system-wide) or 
+   :file:`/home/USER/.../powerline/__init__.pyc` (if powerline was cloned 
+   somewhere, e.g. in :file:`/home/USER/.vim/bundle/powerline`) reports the same 
+   location you use to source in your shell configuration: in first case it 
+   should be some location in :file:`/usr` (e.g. 
+   :file:`/usr/share/zsh/site-contrib/powerline.zsh`), in the second it should 
+   be something like 
+   :file:`/home/USER/.../powerline/bindings/zsh/powerline.zsh`. If this is true 
+   it may be a powerline bug, but if locations do not match you should not 
+   report the bug until you observe it on configuration where locations do 
+   match.
+#) If this problem is observed within the vim instance you should check out the 
+   output of the following Ex mode commands
+
+   .. code-block:: vim
+
+      python import powerline as pl ; print (pl.__file__)
+      python3 import powerline as pl ; print (pl.__file__)
+
+   One (but not both) of them will most likely error out, this is OK. The same 
+   rules apply as in the 1), but in place of sourcing you should seek for the 
+   place where you modify `runtimepath` vim option. If you install powerline 
+   using `VAM <https://github.com/MarcWeber/vim-addon-manager>`_ then no 
+   explicit modifications of runtimpath were performed in your vimrc 
+   (runtimepath is modified by VAM in this case), but powerline will be placed 
+   in :file:`{plugin_root_dir}/powerline` where `{plugin_root_dir}` is stored in 
+   VAM settings dictionary: do `echo g:vim_addon_manager.plugin_root_dir`.
+
+There is a hint if you want to place powerline repository somewhere, but still 
+make powerline package importable anywhere: use
+
+  .. code-block:: shell
+
+     pip install --user --editable path/to/powerline
+
 My vim statusline has strange characters like ``^B`` in it!
 -----------------------------------------------------------
 
@@ -61,14 +120,14 @@ My vim statusline is not displayed completely and has too much spaces
 * Alternative: set :ref:`ambiwidth <config-common-ambiwidth>` to 2, remove fancy 
   dividers (they suck when ``ambiwidth`` is set to double).
 
-When using `z <https://github.com/rupa/z>` powerline shows wrong number of jobs
--------------------------------------------------------------------------------
+When using z powerline shows wrong number of jobs
+-------------------------------------------------
 
-This happens because `z <https://github.com/rupa/z>` is launching some jobs in 
+This happens because `z <https://github.com/rupa/z>`_ is launching some jobs in 
 the background from ``$POWERLINE_COMMAND`` and these jobs fail to finish before 
 powerline prompt is run.
 
 Solution to this problem is simple: be sure that :file:`z.sh` is sourced 
 strictly after :file:`powerline/bindings/bash/powerline.sh`. This way background 
-jobs are spawned by `z <https://github.com/rupa/z>` after powerline has done its 
-job.
+jobs are spawned by `z <https://github.com/rupa/z>`_ after powerline has done 
+its job.