Document how to switch python version for 5.x

Focus is on rebuilding venv (which are bound to a specific python minor
version).

Ref spiral-project#483
Ref spiral-project#571

docs/installation.rst

@@ -1,3 +1,5 @@
+.. _installation:
+
 Installation
 ############
 

docs/upgrade.rst

@@ -35,6 +35,37 @@ Version-specific instructions
 When upgrading from a major version to another, you **must** follow special
 instructions:
 
+4.x → 5.x
+---------
+
+Switch to a supported version of Python
++++++++++++++++++++++++++++++++++++++++
+
+.. note:: If you are already using Python ≥ 3.6, you can skip this section, no
+          special action is required.
+
+If you were running IHateMoney using Python < 3.6, you must, **before** upgrading:
+
+1. Ensure to have a Python ≥ 3.6 available on your system
+2. Rebuild your virtualenv (if any). It will *not* alter your database nor configuration. For example, if your virtualenv is in `/home/john/ihatemoney/`::
+
+     rm -rf /home/john/ihatemoney
+     pyhton3 -m venv /home/john/ihatemoney
+     source /home/john/ihatemoney/bin/activate
+
+  You might need to ``pip install`` additional dependencies if you are using one
+  or several of the following deployment options :
+
+  - Gunicorn (Nginx)
+  - MySQL
+  - PostgreSQL
+
+If so, pick the ``pip`` commands to use in the relevant section(s) of
+:ref:`installation`.
+
+Then follow :ref:`general-procedure` from step 1. in order to complete the update.
+
+
 2.x → 3.x
 ---------