Document experimental Java features

There are a couple of experimental features that are new to try in Avro (Java):

• org.apache.avro.specific.use_custom_coders to use code-generated serialization in SpecificRecords
• org.apache.avro.fastread to use a different deserialization technique reusing logic constructed once from a known schema.

These should be documented someplace (in the code at point of use, wiki page or website?) and noted how to enable them / the risks/benefit you might encounter.