Bugfix - add back logo to the README

[nikhil]

I added back the nf-core tools logo which went missing after version 3

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.89%. Comparing base (496ae45) to head (f7e39ce).
Report is 9 commits behind head on dev.

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[mirpedrol]

Thanks for fixing this!
We should keep the github URL instead of using relative paths, you can use the old one even if it won't work until this PR is merged.

[nikhil]

Thanks! I reverted the relative link update to use the github URL

[mirpedrol]

👏 thank you!