Add pip installation instructions

doc/sphinx/source/quickstart/installation.rst

@@ -6,12 +6,16 @@ Installation
 
 ESMValTool 2.0 requires a Unix(-like) operating system and Python 3.6+.
 
-The ESMValTool supports three different installation methods:
+The ESMValTool supports five different installation methods:
 
 * Installation through Conda package manager (see https://www.continuum.io);
 
+* Installation with Pip and Conda package manager (see https://pypi.org);
+
 * Deployment through a Docker container (see https://www.docker.com);
 
+* Deployment through a Singularity container (see https://sylabs.io/guides/latest/user-guide/);
+
 * From the source code available at https://github.com/ESMValGroup/ESMValTool.
 
 The next sections will detail the procedure to install ESMValTool for each of
@@ -21,11 +25,14 @@ this methods.
 Conda installation
 ==================
 
-In order to install the Conda package, you will need both conda and Julia
-pre-installed, this is because Julia cannot be installed from conda.
-For a minimal conda installation go to https://conda.io/miniconda.html.
+In order to install the `Conda <https://docs.conda.io>`_ package, you will need
+both Conda and Julia pre-installed, this is because Julia cannot be installed
+from Conda.
+For a minimal conda installation (recommended) go to https://conda.io/miniconda.html.
+It is recommended that you always use the latest version of Conda, as problems
+have been reported when trying to use older versions.
 Installation instructions for Julia can be found on the
-`Julia download page <https://julialang.org/downloads/>`_.
+`Julia installation instructions page <https://julialang.org/downloads/platform/>`_.
 
 Once you have installed the above prerequisites, you can install ESMValTool by running:
 
@@ -34,8 +41,28 @@ Once you have installed the above prerequisites, you can install ESMValTool by r
     conda install esmvaltool -c esmvalgroup -c conda-forge
 
 Here ``conda`` is the executable calling the Conda package manager to install
-``esmvaltool`` and the ``-c`` flag specifies the software channels in which ``esmvaltool``
-and its dependencies can be found.
+``esmvaltool`` and the ``-c`` flag specifies the Conda software channels in which the
+``esmvaltool`` package and its dependencies can be found.
+
+It is also possible to create a new
+`Conda environment <https://docs.conda.io/projects/conda/en/latest/user-guide/getting-started.html#managing-environments>`_
+and install ESMValTool into it with a single command:
+
+.. code-block:: bash
+
+    conda create --name esmvaltool -c esmvalgroup -c conda-forge esmvaltool
+
+Don't forget to activate the newly created environment after the installation:
+
+.. code-block:: bash
+
+    conda activate esmvaltool
+
+Of course it is also possible to choose a different name than ``esmvaltool`` for the environment.
+
+.. note::
+
+	  Creating a new Conda environment is often much faster and more reliable than trying to update an existing Conda environment.
 
 Installation of subpackages
 ---------------------------
@@ -66,6 +93,39 @@ Note that it is only necessary to install Julia prior to the conda installation
 
 Note that the ESMValTool source code is contained in the ``esmvaltool-python`` package, so this package will always be installed as a dependency if you install one or more of the packages for other languages.
 
+Pip installation
+================
+
+It is also possible to install ESMValTool from `PyPI <https://pypi.org/project/ESMValTool/>`_.
+However, this requires first installing dependencies that are not available on PyPI in some other way.
+By far the easiest way to install these dependencies is to use conda_.
+For a minimal conda installation (recommended) go to https://conda.io/miniconda.html.
+
+After installing Conda, download
+`the file with the list of dependencies <https://raw.githubusercontent.com/ESMValGroup/ESMValTool/master/environment.yml>`_:
+
+.. code-block:: bash
+
+    wget https://raw.githubusercontent.com/ESMValGroup/ESMValTool/master/environment.yml
+
+and install these dependencies into a new conda environment with the command
+
+.. code-block:: bash
+
+    conda env create --name esmvaltool --file environment.yml
+
+Finally, activate the newly created environment
+
+.. code-block:: bash
+
+    conda activate esmvaltool
+
+and install ESMValTool as well as any remaining dependencies with the command:
+
+.. code-block:: bash
+
+    pip install esmvaltool
+
 Docker installation
 ===================
 
@@ -320,5 +380,3 @@ confirm that no errors are reported:
 .. code-block:: bash
 
     python setup.py test
-
-