Add a simple introduction to the environments section

Software_Reference/Environments.rst

@@ -3,6 +3,13 @@
 Environments
 ============
 
+The Librem 5 is able to run two different phone environments for applications:
+phosh is developed at Purism as a mobile-friendly replacement for the
+GNOME shell; Plasma Mobile is a project from the KDE project.
+
+The following sections describe these environments in more detail, covering
+the steps needed to build and install each of them.
+
 .. toctree::
 
    Environments/Phosh