Add overview text

Author: bouweandela

_episodes/02-installation.md

@@ -14,6 +14,15 @@ keypoints:
 ---
 ## Overview
 
+In this tutorial we will be using the [Conda](https://conda.io/projects/conda/en/latest/index.html)
+package manager to install the ESMValTool.
+Other installation methods are also available, they can be found in the
+[documentation](https://docs.esmvaltool.org/en/latest/quickstart/installation.html).
+ESMValTool also contains diagnostics written in [Julia](https://julialang.org/).
+Because Julia cannot be installed by Conda, we will install Julia separately.
+We will first learn how to install Conda, Julia, and finally the ESMValTool.
+We end this chapter by testing that the installation was successful.
+
 ## Install Conda
 
 ## Install Julia