Docs: Add instruction on how to install the package

Author: sphuber

docs/source/index.rst

@@ -54,6 +54,49 @@ The common workflows can be subdivided into two categories:
       :classes: btn-outline-primary btn-block stretched-link
 
 
+.. _installation:
+
+************
+Installation
+************
+
+The Python package can be installed from the Python Package index (PyPI) or directly from the source:
+The recommended method of installation is to use the Python package manager ``pip``:
+
+.. code-block:: console
+
+    $ pip install aiida-common-workflows
+
+This will install the latest stable version that was released to PyPI.
+Note that this will not install any of the plugin packages that are required to run any of the common workflow implementations.
+To install all plugin packages that implement a common workflow, run the install with the ``all_plugins`` extra:
+
+.. code-block:: console
+
+    $ pip install aiida-common-workflows[all_plugins]
+
+Alternatively, you can choose a specific plugin to prevent having to install all plugin packages, for example:
+
+.. code-block:: console
+
+    $ pip install aiida-common-workflows[quantum_espresso]
+
+will install the package plus the dependencies that are required to run the implementation for Quantum ESPRESSO.
+The available extras are ``abinit``, ``bigdft``, ``castep``, ``cp2k``, ``fleur``, ``gaussian``, ``gpaw``, ``nwchem``, ``orca``, ``quantum_espresso``, ``siesta``, ``vasp`` and ``wien2k``.
+
+To install the package from source, first clone the repository and then install using ``pip``:
+
+.. code-block:: console
+
+    $ git clone https://github.com/aiidateam/aiida-common-workflows
+    $ pip install -e aiida-common-workflows
+
+The ``-e`` flag will install the package in editable mode, meaning that changes to the source code will be automatically picked up.
+
+To work with ``aiida-common-workflows``, a configured AiiDA profile is required.
+Please refer to `AiiDA's documentation <https://aiida.readthedocs.io/projects/aiida-core/en/latest/intro/get_started.html>`_ for detailed instructions.
+
+
 .. _how-to-submit:
 
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