Add an important note to master

[bitfort]

Most MLPerf repositories use master as their source of truth. We should either migrate to that or clearly indicate not to use this branch.

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[arjunsuresh]

Thank you @bitfort for this suggestion. Actually our plan was indeed to use the master branch for submission until this issue came and the related emergency meeting which necessitated a move to r3.0 branch for power. This was announced in the meetings (both power and inference) and also the power-dev git submodule of the inference repository was updated to r3.0 branch. But I understand that such announcements may not be reaching each and every developer working for different submitters and we have seen this happen previously too. In fact this is precisely why inference WG moved to using the master branch for submission and power-dev can also do the same from next rounds. But even then any submitter must be using a valid commit hash (or later) and I guess this check we can add as soon as we have that valid commit hash for 3.1 and so on.