Add note about auditing being unsupported

[JamesMcMahon]

Based off of #281

The goal is to increase the visibility of unsupported features to enable devs to make smarter choices about the tradeoffs currently needed for R2DBC support.

Let me know what you think. I personally wasted some time trying to Auditing to work before I saw the aforementioned issue. Having a quick summary of features the authors know is not currently implemented could be useful for people using R2DBC for the first time.

[mp911de]

That's not how it works. The list of unsupported features would otherwise grow with every feature that is invented in a different module. Instead, we document features that are supported.

[JamesMcMahon]

Thanks for taking a look at the PR. I figured it was easier to just make the PR then ask if you were open to one and then make it.

I would argue that there is some value in documenting what standard features of the Spring Data ecosystem are not currently supported.

I really like what you have over on Postgres R2DBC README with the table that lays out supported and unsupported data type, https://github.com/r2dbc/r2dbc-postgresql#data-type-mapping.

point	Not yet supported.
polygon	Not yet supported.
real	Float, Double, Boolean, Byte, Short, Integer, Long, BigDecimal, BigInteger
smallint	Short, Boolean, Byte, Integer, Long, BigDecimal, BigInteger

Maybe the list of Spring Data is too long to do something comparable. I also see what you are saying about modules introducing new features.

[mp911de]

Postgres comes with a pretty stable set of data types (not considering extensions) and changes occur rarely. Spring Data modules support a variety of things (Query by Example, Querydsl, Schema generation, Auditing, lifecycle events, Entity callbacks, @Value in constructors, fluent Template API, …) and that list is growing over time. With over 15 modules and several of them being maintained by the community it's not feasible to keep each documentation up to date about the not supported things.