doc: mentioned the bitop library dependency when using the standard Lua 5.1 interpreter. thanks Laurent Arnoud for the patch in #9.

Author: agentzh

README.markdown

@@ -536,10 +536,13 @@ It is recommended to use the latest [ngx_openresty bundle](http://openresty.org)
 is bundled and enabled by default. At least ngx_openresty 1.4.2.9 is required. And you need to enable LuaJIT when building your ngx_openresty
 bundle by passing the `--with-luajit` option to its `./configure` script. No extra Nginx configuration is required.
 
-If you want to use this library with your own Nginx build (with ngx_lua), then you need to
-ensure you are using at least ngx_lua 0.9.0. Also, You need to configure
-the [lua_package_path](https://github.com/chaoslawful/lua-nginx-module#lua_package_path) directive to
-add the path of your lua-resty-websocket source tree to ngx_lua's Lua module search path, as in
+If you want to use this library with your own Nginx build (with ngx_lua), then
+you need to ensure you are using at least ngx_lua 0.9.0
+(and [lua-bitop](http://bitop.luajit.org/) library if you are not using LuaJIT).
+Also, You need to configure the
+[lua_package_path](https://github.com/chaoslawful/lua-nginx-module#lua_package_path)
+directive to add the path of your lua-resty-websocket source tree to ngx_lua's
+Lua module search path, as in
 
 ```nginx
     # nginx.conf