Add note about being in alpha

Author: stevenjmesser

source/index.html.md.erb

@@ -5,6 +5,8 @@ weight: 1
 
 # About these docs
 
+<%= warning_text('This is a prototype.') %>
+
 This is a reference guide for who we are and the way we do things. It's a playbook for anyone in the GOV.UK Design System team that tells us the 'right way' to do things.
 
 What's written here is the way we do things now, but that will change from time to time.