etc/csl: Add a README in preparation for Emacs sync

Following nearly all other subdirectories in Emacs's etc/, add a
README explaining what the files are and what license they are under.

I did not add these files, but I have verified that

 - etc/csl/chicago-author-date.csl is identical to
   9821a884d2cbf919a064426999b05aa78e98da8d:chicago-author-date.csl
   from https://github.com/citation-style-language/styles

 - etc/csl/locales-en-US.xml is identical to
   a2b76b20ee120979ec66cfdcfa798b36fc5a7f0b:locales-en-US.xml from
   https://github.com/citation-style-language/locales

etc/csl/README

@@ -0,0 +1,10 @@
+These data files are used by Org's oc-csl.el library.
+
+COPYRIGHT AND LICENSE INFORMATION
+
+chicago-author-date.csl
+locales-en-US.xml
+
+  Both of these files are part of the Citation Style Language (CSL)
+  project (<https://citationstyles.org/>) and are released under the
+  Creative Commons Attribution-ShareAlike 3.0 Unported license.