Update readme

{{cookiecutter.project_slug}}/README.rst

@@ -10,3 +10,33 @@
 .. image:: https://git.km3net.de/examples/km3badges/-/raw/master/docs-latest-brightgreen.svg
     :target: https://{{cookiecutter.gitlab_username}}.pages.km3net.de/{{cookiecutter.project_slug}}
 
+
+Installation
+~~~~~~~~~~~~
+
+It is recommended to first create an isolated virtualenvironment to not interfere
+with other Python projects::
+
+  git clone {{cookiecutter.gitlab_repo_path}}
+  cd {{cookiecutter.project_slug}}
+  python3 -m venv venv
+  . venv/bin/activate
+
+Install directly from the Git server via ``pip`` (no cloneing needed)::
+
+  pip install git+{{cookiecutter.gitlab_repo_path}}
+
+Or clone the repository and run::
+
+  make install
+
+To install all the development dependencies, in case you want to contribute or
+run the test suite::
+
+  make install-dev
+  make test
+
+
+---
+
+*Created with ``cookiecutter https://git.km3net.de/templates/python-project``*