Add note about creating src directory

Author: weavejester

index.adoc

@@ -603,7 +603,19 @@ we have no routes defined. The error page will be in plaintext, because
 we haven't specified what _features_ we want for our web application.
 
 We'll fix both these issues, but before we do we should terminate the
-application with Ctrl-C and start a REPL instead.
+application with Ctrl-C and make a `src` directory.
+
+[,shell]
+----
+mkdir src
+----
+
+IMPORTANT: The source directory must be created before running the REPL.
+This is because Clojure looks for source files in the Java classpath,
+and directories that don't exist yet will be omitted from this.
+
+Then we'll start a REPL. We'll keep this running while we develop the
+application.
 
 [,shell]
 ----