[DOC] Installation guide

INSTALL.md

-To bootstrap gargantext environment just do:
+# Gargantext installation and dev environment
 
-    pipenv install
+## TL;DR
+
+You need `pipenv` and an up-to-date version of `pip`. To install and run:
+
+    $ pipenv install
+    $ pipenv run ./manage runserver
+
+
+## Requirements
+
+### Up-to-date pip
+
+On Debian-like distros, `pip` is not installed by default, if you didn't do it
+already:
+
+    $ sudo apt install python3-pip
+
+Pipenv (see below) needs an up-to-date version of pip, on a Debian-like just
+`apt upgrade`, otherwise upgrade it like so:
+
+    $ pip install pip --user --upgrade
+
+### Pipenv
+
+You will need [pipenv][1] to easily get dependencies of Gargantext.
+It handles packages and takes care of the virtualenv and environment variables.
+
+There are various ways to install `pipenv`, see its [documentation][2] for more
+insights. Here is the straightforward way:
+
+    $ pip install pipenv --user
+
+[1]: https://github.com/kennethreitz/pipenv
+[2]: https://docs.pipenv.org/
+
+
+## Installation
+
+To bootstrap Gargantext environment just cd into your local Gargantext repo and
+do:
+
+    $ pipenv install
+
+If everything is going well, you now have a clean virtualenv with every
+packages you need to run Gargantext. Here is how to get the path of the
+virtualenv directory pipenv just created:
+
+    $ pipenv --venv
+
+You can now run any command by prefixing it with `pipenv run` or by first
+entering the virtualenv with `pipenv shell`. To run Gargantext django backend
+test server you can do:
+
+    $ pipenv run ./manage.py runserver
+
+But also:
+
+    $ pipenv shell
+    $ ./manage.py runserver
+    $ ... continue in your virtualenv ...
+
+
+## Customize dev environment
+
+To install specific packages without messing with dependencies, just use pip.
+For example, to install ipython or bpython shells locally:
+
+    $ pip install ipython
+    $ pip install bpython