add more detail to the readme

Author: zgulde

README.md

@@ -1,3 +1,13 @@
+# Codeup Setup Script
+
+Copy and paste the following in your terminal:
+
 ```
 bash -c "$(curl -sS https://raw.githubusercontent.com/zgulde/codeup-setup-script/master/install.sh)"
 ```
+
+## Note for Instructors
+
+If students already have and `id_rsa` ssh key generated the script will *not* try to
+generate a new ones, and you will need to walk them through the process of adding
+their existing key to Github.