Fix ex_doc warnings

Author: ericmj

rebar.config

@@ -19,7 +19,8 @@
 {ex_doc, [
     {source_url, <<"https://github.com/hexpm/hex_core">>},
     {extras, [<<"README.md">>, <<"CHANGELOG.md">>]},
-    {main, <<"readme">>}
+    {main, <<"readme">>},
+    {skip_undefined_reference_warnings_on, [<<"CHANGELOG.md">>]}
 ]}.
 
 {hex, [{doc, #{provider => ex_doc}}]}.