Readme: Explain that references don't respect the underscore convention

[tordans]

No description provided.

[tordans]

@tyrasd +1 to merge this or something similar so issues like openstreetmap/id-tagging-schema#1415 (comment) don't have to be researched …

[k-yle]

relevant code:

schema-builder/lib/build.js

Lines 296 to 300 in 54c2a7d

	function stripLeadingUnderscores(str) {
	return str.split('/')
	.map(s => s.replace(/^_/,''))
	.join('/');
	}

[tordans]

I see that I have merge permissions here for my own PRs and I am using this power now to merge this, given that I already created two PRs for the same thin apparently and had to research it again yesterday …

The PR was approved and it is only a documentation change so I think that is OK two work around the rules we set ourselves in the id-taggign-schema repo.