doc(README): update render-project and delete-project-videos examples.

Author: ArtyomHov

README.md

@@ -216,7 +216,7 @@ const renderforest = new Renderforest({ signKey: '<signKey>', clientId: -1 })
 
 const payload = {
   projectId: 5000658,
-  quality: 360
+  quality: 360 // optional argument
 }
 renderforest.deleteProjectVideos(payload)
   .then(console.log) // handle the success
@@ -271,7 +271,8 @@ const renderforest = new Renderforest({ signKey: '<signKey>', clientId: -1 })
 
 const payload = {
   projectId: 5000658,
-  quality: 1080
+  quality: 360,
+  watermark: 'https://example.com/watermark.png' // optional argument
 }
 renderforest.renderProject(payload)
   .then(console.log) // handle the success