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 +1,5 @@
+The bibliopgrahic data available, which is also accessible via
+https://xmpp.org/extensions/refs/, is available openly and free of
+charge under the CC BY-ND 4.0 [1] license.
+
+1: https://creativecommons.org/licenses/by-nd/4.0/