doc - add extensions option

Author: haoxin

Readme.md

@@ -33,6 +33,7 @@ app.use(require('koa-static')(root, opts));
  - `index` Default file name, defaults to 'index.html'
  - `defer` If true, serves after `return next()`, allowing any downstream middleware to respond first.
  - `gzip`  Try to serve the gzipped version of a file automatically when gzip is supported by a client and if the requested file with .gz extension exists. defaults to true.
+ - `extensions` Try to match extensions from passed array to search for file when no extension is sufficed in URL. First found is served. (defaults to `false`)
 
 ## Example