Update of the documentation

Author: funilrys

docs/installation/get-domain2idna.rst

@@ -6,6 +6,20 @@ Using :code:`pip`
 
 Choose your repository, install and enjoy domain2idna!
 
+From PyPi
+""""""""""
+
+::
+
+   $ pip3 install --user domain2idna
+
+.. note::
+   We recommend the :code:`--user` flag which installs the required dependencies at the user level. More information about it can be found on `pip documentation`_.
+.. warning::
+   We do not recommend the :code:`--user` flag when using :code:`domain2idna` into containers like - for example - Travis CI.
+
+.. _pip documentation: https://pip.pypa.io/en/stable/reference/pip_install/?highlight=--user#cmdoption-user
+
 From GitHub
 """""""""""