[openrailwaymap] Tagging proposal Dutch ETCS stop markers

Dear contributors, The Dutch national law (https://wetten.overheid.nl/BWBR0017707/2020-04-01 appendix 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