SnpEff installation instructions

[HolEv]

Description of feature

add instructions on how to install snpeff (esp. how to set up genome versions/downloads)

[LArnoldt]

It's basically just a conda installation. It's mentioned in the pyproject.toml file and I have also added it in the explore_annotations.iypnb. The reason for the database download issue is a different one though: pcingola/SnpEff#602 - I will monitor this and message when this is resolved. The recent commit messages indicate that the databases are indeed moved to AWS Open Data and there was an issue 2 days ago who prepare updating the conda package. I think it should be resolved relatively soon.