Updated README

Author: wapacro

README.md

@@ -202,6 +202,7 @@ $defaultOptions = [
     'urgency' => 'normal', // protocol defaults to "normal". (very-low, low, normal, or high)
     'topic' => 'newEvent', // not defined by default. Max. 32 characters from the URL or filename-safe Base64 characters sets
     'batchSize' => 200, // defaults to 1000
+    'contentType' => 'application/json', // defaults to "application/octet-stream"
 ];
 
 // for every notification
@@ -235,6 +236,11 @@ In order to fix this, WebPush sends notifications in batches. The default size i
 to decrease this number. Do this while instantiating WebPush or calling `setDefaultOptions`. Or, if you want to customize this for a specific flush, give
 it as a parameter : `$webPush->flush($batchSize)`.
 
+#### contentType
+
+Sets the "Content-Type" header for HTTP requests with a non-empty payload sent to the push service. 
+Especially newer [Declarative push messages](https://www.w3.org/TR/push-api/#declarative-push-message) require a specific JSON payload, so this should be set to "application/json" in such cases.
+
 ### Server errors
 
 You can see what the browser vendor's server sends back in case it encountered an error (push subscription expiration, wrong parameters...).