Add context on MARS data collection to streamline data reviews

[abedef]

Abed and I added a few additional sections to our README outlining MARS data collection practices to give background and context for data reviewers. This is part of an effort to make data reviews easier and faster for data reviewers and ads engineers.

The idea would be to share a link to this README whenever requesting a data review, so if a reviewer is new to MARS's use of Glean, they can read it first before engaging with the review. I added a note to the data review request instructions for that bit.

[abedef]

Thanks for writing this up, Masha! Looks quite clear to me now. Hopefully it does the trick for our future data reviews 👍

(Leaving unapproved so someone else can have a look to make sure everything is accurate and that this isn't mentioning any non-public info)

[symhom]

processes them to redact or anonymize our users' information

This implies that user information is being sent from the firefox browser to Mars. Maybe list out all the PII sent from the firefox request so people don't assume emails are also part of the request.

[mashalifshin]

Ooh yes I see how it could read that way, thanks for pointing it out. Could you take a look at my updates and lmk what you think?

(I wanted to avoid getting into listing specific things, since in our case it's not really about the info explicitly being passed, so even implying that was part of the problem you identified -- its more about what is fingerprintable from the standard http request info, so I tried to reword around that ...)

[efixler]

💥