Fix some out-of-date instructions in /_ref/README, and clarify /Readme.

Author: rocketnia

Readme

@@ -1,4 +1,5 @@
 Sources for the arc community site at http://arclanguage.github.io.
 
 The arc 3.1 reference at ref/ is generated from _ref/ using the
-_ref/generate.sh script.
+_ref/generate.sh script. The script should be run from the _ref/
+directory.

_ref/README

@@ -3,8 +3,8 @@ template files, run:
 
 $ ./generate.sh
 
-The resulting output will appear in the output/ directory.  You can
-open the output/index.html file directly in your web browser; you
+The resulting output will appear in the ref/ directory.  You can
+open the ref/index.html file directly in your web browser; you
 don't need to be running a web server.
 
 When you're happy with your changes, push to github to update