Add social preview image

[maxulysse]

• Add social preview image

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 necessary, also make a PR on the nf-core/atacseq branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
• Documentation in docs is updated
• CHANGELOG.md is updated
• README.md is updated

Learn more about contributing: https://github.com/nf-core/atacseq/tree/master/.github/CONTRIBUTING.md

[drpatelh]

What happened to the fancy custom ones you created for me? Cant see them anymore in my Slack history 😓

[maxulysse]

You meant the logos?
As we're not paying for Slack, so history is limited.
Currently, as there is no pipeline specific logo, this is the social preview image.
When there is logo, we can talk about an updated social preview

[drpatelh]

Tests are failing due to some weird channel error with the latest version of Nextflow. Quite an uncontroversial change so Ill merge and fix downstream. Thanks @maxulysse 😎