Note required CBMi SnpEff annotations when loading

This updates the README to include a note warning the user that any VCFs being loaded into Varify currently must be annotated using the CBMi SnpEff fork(https://github.com/CBMi-BiG/snpEff) to run through the loader.

Signed-off-by: Don Naegely <naegelyd@gmail.com>