README.md

The Variant spec is being developed in the Apache Parquet project, and has not been finalized. Spark currently implements the version of the spec at https://github.com/apache/parquet-format/blob/37b6e8b863fb510314c07649665251f6474b0c11/VariantEncoding.md and https://github.com/apache/parquet-format/blob/37b6e8b863fb510314c07649665251f6474b0c11/VariantShredding.md, but does not yet support Variant values containing UUID, Time, or nanosecond-precision Timestamp. Spark does not yet have public APIs to enable shredded writes.