Clarify that commented code is for local testing

[FloEdelmann]

Like it is done e.g. here:

streetcomplete-mapstyle/streetcomplete-dark-style.yaml

Lines 1 to 13 in c9ad090

	# for online testing (e.g. Tangram Play)
	import:
	- https://raw.githubusercontent.com/streetcomplete/streetcomplete-mapstyle/jawg/global.yaml
	- https://raw.githubusercontent.com/streetcomplete/streetcomplete-mapstyle/jawg/layers/land.yaml
	- https://raw.githubusercontent.com/streetcomplete/streetcomplete-mapstyle/jawg/layers/labels.yaml
	- https://raw.githubusercontent.com/streetcomplete/streetcomplete-mapstyle/jawg/layers/roads.yaml
	# for local testing (e.g. using an emulator)
	#import:
	# - global.yaml
	# - layers/land.yaml
	# - layers/labels.yaml
	# - layers/roads.yaml

Note: this PR is based on #127, as I didn't want to introduce merge conflicts.