Clarify that POST requests can work using the CORS feature (#208)

Author: mattab

docs/3.x/tracking-javascript.md

@@ -112,7 +112,7 @@ Piwik uses first party cookies to keep track of some user information over time.
 ### Advanced uses
 
 *   `addListener( element )` - Add click listener to a specific link element. When clicked, Piwik will log the click automatically.
-*   `setRequestMethod( method )` - Set the request method to either "GET" or "POST". (The default is "GET".) To use the POST request method, the Piwik host must be the same as the tracked website host (Piwik installed in the same domain as your tracked website).
+*   `setRequestMethod( method )` - Set the request method to either "GET" or "POST". (The default is "GET".) To use the POST request method, either 1) the Piwik host is the same as the tracked website host (Piwik installed in the same domain as your tracked website), or 2) if Piwik is not installed on the same host as your website, you need to [enable CORS (Cross domain requests) as explained in this FAQ](https://piwik.org/faq/how-to/faq_18694/).
 *   `setCustomRequestProcessing( function )` - Set a function that will process the request content. The function will be called once the request (query parameters string) has been prepared, and before the request content is sent.
 *   `setRequestContentType( contentType )` - Set request Content-Type header value. Applicable when "POST" request method is used via `setRequestMethod`.