Merge pull request #2704 from square/jwilson.0320.be_sad_about_jaxb

Document that JAXB doesn't work on Android

retrofit-converters/jaxb/README.md

@@ -6,6 +6,7 @@ A `Converter` which uses [JAXB][1] for serialization to and from XML.
 A default `JAXBContext` instance will be created or one can be configured and passed
 to `JaxbConverterFactory.create()` to further control the serialization.
 
+**Note that JAXB does not work on Android.**
 
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