Documented the linked command

documentation/sitefiles.md

@@ -15,6 +15,7 @@ sitefiles also allow comments with #
 * ```define [variable] [value]``` - sets some global variable
 * ```read [http path] [file path]``` - if the requested path matches ```[http path]```, return the contents of ```[file path]```. If [file path] is a directory, then the http path is appended to [file path] and that is read instead.
 * ```exec [http path] [file path]``` - if the requested path matches ```[http path]```, execute ```[file path]``` and send the the contents of stdout. ```argv[0]``` is the requested path, ```argv[1]``` is a header, ```argv[2]``` is the value for that header, ```argv[3]``` is the next header, and so on.
+* ```linked``` - Run getResponse() from the library loaded from the library global variable
 * ```throw [http path] [error code]``` - If the requested path matches ```[http path]```, send back the http error code ```[error code]```. For standardization purposes, these error codes are just the number.
 
 ##### Other than set, commands should take in a regex as argument 1 and operate on a file specified in argument 2.