[openrailwaymap] Tagging proposal Dutch ETCS stop markers

Dear contributors,

The Dutch national law (https://wetten.overheid.nl/BWBR0017707/2020-04-01%C2%A0appendix 2, chapter 13) allows two types of ETCS stop marker signs under the same signal reference number:

This makes tagging a bit harder than usual. Normally we would just say: railway:signal:train_protection = nl:227b

Because there are two types of signs I propose the tags:

railway:signal:train_protection = nl:227b_triangle railway:signal:train_protection = nl:227b_arrow

In case we agree about this, I will proceed with changing the wiki and my (currently 'paused') pull request.

Best regards, JJJWegdam