Add note about required toolbar button image size

[jacekbogdanski]

Plugins with images highly exceeding 16x16 px results in broken build. Contributors should be aware of the required size of the image.

Error message from the builder with incorrect icon size:

cowardly refused to add an image to a sprite because it’s too big

Let's add a short information to the plugin contribution page about requirements, e.g.:

Make sure your button icon is in the right dimensions 16x16px.

[vokiel]

Moved to the right repository cksource/ckeditor.com#521