add small documentation, also raise with a useful message if they require mysqlplus after mysql--don't know if that works with 1.9 but it does with 1.8.x

lib/mysqlplus.rb

@@ -1,5 +1,9 @@
-require 'mysql'
+require 'mysql' # this should load the mysqlplus version of mysql.so, as we assume the user has installed mysql as a gem and have not done any previous "require 'mysql'" to have loaded the other
 
+#
+# Mysqlplus library gives you a [slightly modified] version of the Mysql class
+# See http://www.kitebird.com/articles/ruby-mysql.html for details, as well as the test directory within the library
+#
 class Mysql
   
   def ruby_async_query(sql, timeout = nil) # known to deadlock TODO
@@ -8,7 +12,10 @@ class Mysql
     get_result
   end
 
-  alias_method :async_query, :c_async_query  
+  begin
+    alias_method :async_query, :c_async_query  
+  rescue NameError => e
+    raise LoadError.new "error loading mysqlplus--this may mean you ran a require 'mysql' before a require 'mysqplus', which much come first"
+  end
   
 end
-