Add MacStadium sponsorship to README

[maxichrome]

MacStadium are providing us with free Apple silicon compute under their Open Source Developer program. This PR adds a segment acknowledging them for doing so.

I'd like to pass the provided text by Jamie and the other requested reviewers before merging upstream and submitting to MacStadium for review.

Closes ENG-273

[linear[bot]]

ENG-273 Add MacStadium sponsorship to README

According to MacStadium the requirements are as follows:

Please put the MacStadium Open Source Developer logo on your landing page (either your GitHub page or website).

I created a custom light- and dark-theme SVG of this logo and have applied it to our README.

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[maxichrome]

Moving this to draft status while we wait for MacStadium's confirmation

[maxichrome]

MacStadium never got back to me on this. I'm not sure if we're actually going to receive this. Closing until I hear otherwise from their team.