Add license for bibliographic data

We wheref asked [1] to state that the XEP bibliographic data is openly
available and free of charge if we want it to be consumed by third
parties.

Fixes xsf#1219.

1: ietf-tools/bibxml-service#302 (comment)

Makefile

@@ -76,8 +76,11 @@ inbox-xml: $(OUTDIR)/inbox $(proto_xep_xmls)
 .PHONY: pdf
 pdf: $(xep_pdfs)
 
+$(REFSDIR)/LICENSE: refs-LICENSE
+	cp $< $@
+
 .PHONY: refs
-refs: $(xep_refs)
+refs: $(xep_refs) $(REFSDIR)/LICENSE
 
 .PHONY: examples
 examples: $(xep_examples)

refs-LICENSE

@@ -0,0 +1,9 @@
+To the extent possible under law, the XMPP Standards Foundation (XSF)
+has waived all copyright and related or neighboring rights to XSF's
+bibliographic data.
+
+That is, the bibliographic data, which is also accessible via
+https://xmpp.org/extensions/refs/, is put in the public domain via
+CC0 1.0 [1].
+
+1: https://creativecommons.org/publicdomain/zero/1.0/