Added the command to install babel-preset for babel-loader

While testing this bundle i was getting hit by an erorr that i can't find the babel preset es2017 which i solved it by installing.
I'm new to this webpack thing and to work like this with javascript. Some people might be in my position and try for hours to solve this. :D

Author: bogdaniel

frontend/encore/babel.rst

@@ -7,6 +7,10 @@ Configuring Babel
 
 Need to extend the Babel configuration further? The easiest way is via
 ``configureBabel()``:
+But first let's install the babel preset.
+
+``yarn add babel-preset-es2017``
+This will install the preset that we will use later to configure the babel.
 
 .. code-block:: javascript