Prepare for release 0.3-beta

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
[ ] Make sure your code lints (nf-core lint).
[ ] Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
[ ] Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
[ ] 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).