Point out that yMMSL files are YAML

[LourensVeen]

It wasn't clear to everyone that yMMSL files are YAML files, and it helps to know that if you want to type them! So that should be pointed out. Also, if you call them .ymmsl then most editors won't realise it's YAML, and won't syntax-highlight stuff or help you make the file, and many people are used to their IDE assisting them. So maybe we should also point out that you can call them .yaml and get these features? MUSCLE3 doesn't care about the extension.