Mod: add post way to receive parameter and let user can vist the file in webroot

you need to vist the /filter  path to call the filter server,whether through GET OR POST.
Mod: add the demo page to show the service by post method

a simple form page with a input named word,and the action of form is /filter
Mod:update RREAME.md to adapter the change of filter_server

add the info of how to use this service by post method,and the path of demo page