Add section in readme about current mappings. [DATAJDBC-156]

**[Jens Schauder](https://jira.spring.io/secure/ViewProfile.jspa?name=schauder)** opened **[DATAJDBC-156](https://jira.spring.io/browse/DATAJDBC-156?redirect=false)** and commented

Should include

* what works
* how does it work
* why does it work this way



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**Referenced from:** pull request https://github.com/spring-projects/spring-data-jdbc/pull/26
