Update readme about required API key

[rvdsteege]

The readme states:

wp-pronamic-google-maps/readme.txt

Lines 25 to 29 in b059173

	* **Google Maps API version 3**
	Most of the available plugins still use the Google Maps version 2 API. To use the Google Maps v2 API you
	have to require an API key and include this in the plugin configuration. The Pronamic Google Maps plugin
	uses the Google Maps version 3 API. Wich no longer requires an API key, so you don't have
	to configure this plugin.

However, as noted in https://wordpress.org/support/topic/maps-tab-at-edit-post-doesnt-work/:

De map die tevoorschijn komt bij “Bewerk bericht” geeft de melding “Google Maps kan niet correct geladen worden op deze pagina”.

[…]

Vraag 2: In de info over de plugin word genoemd dat je geen API-key nodig hebt, wat is de reden dat het veld voor API-key er nog wel is in de plugin?

AFAIK, all Google Maps integrations require an API key, thus the readme is incorrect and should be updated.