add instructions for creating databases

PR checklist

[ ] This comment contains a description of changes (with reason).
[ ] If you've fixed a bug or added code that should be tested, add tests!
[ ] If you've added a new tool - have you followed the pipeline conventions in the contribution docs- [ ] If necessary, also make a PR on the nf-core/blobtoolkit branch on the nf-core/test-datasets repository.
[ ] Make sure your code lints (nf-core lint).
[ ] Ensure the unit test suite passes (nextflow run . -profile test,singularity,sanger --outdir <OUTDIR>) locally.
[ ] Ensure the full test suite passes (nextflow run . -profile test_full,singularity,sanger --outdir <OUTDIR>) locally.
[ ] Ensure that the CI tests pass by checking the nextflow tower link at the bottom of the CI tests and check the output on farm5 (the CI test only tells you if the job was successfully submitted to nextflow tower).
[x] Usage Documentation in docs/usage.md is updated.
[ ] Output Documentation in docs/output.md is updated.
[ ] CHANGELOG.md is updated.
[ ] README.md is updated (including new tool citations and authors/contributors).