Add a note about the nightly the guide was written for

rust-lang/rust#31487 included breaking changes
which cause codegen to no longer compile on the latest nightly. It will
likely be several days before I can synchronize everything to work on
both nightly and stable against the latest versions.

source/views/guides/getting-started.html.slim

@@ -21,6 +21,11 @@ main
           Rust in the final chapter. Before we start, make sure you have
           PostgreSQL installed and running.
 
+          Note: Diesel 0.5.0 currently compiles against nightly as of
+          `2016-02-09`. If you're following along with this guide, make sure
+          you're using a nightly older than that version, or add the steps in
+          the final section to compile on stable
+
           The first thing we need to do is generate our project.
 
         .demo__example