Adds Marketplace link to README

[Stanzilla]

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Description

Checklist

• I have followed the guidelines in the Contributing document
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[SethFalco]

Hey! Just wanted to bump this and say it's probably a good call to add it to the README.
I wasn't sure if the extension was still being published or not until I saw this. ^-^'

Edit: Ahh, whoops, wasn't meant to approve. The extension name/ID was renamed, so this PR just needs to be updated to reflect that. ^-^'

[SethFalco]

(Also has conflicts btw!)