Explain the PyTorch SEV process

What-is-considered-a-SEV?.md

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+[File a SEV](https://github.com/pytorch/pytorch/issues/new?assignees&labels=ci%3A+sev&template=ci-sev.md)
+
+# Why do we [file SEVs](https://github.com/pytorch/pytorch/issues/new?assignees&labels=ci%3A+sev&template=ci-sev.md)?
+
+1. Alert devs that something is currently wrong with the CI and they may see failures that they’re not responsible for
+2. Track developer facing issues so that we can appropriately prioritize fixing (without SEVs these can get forgotten)
+
+# What’s considered a SEV?
+
+When there is a widespread CI breakage that is not resolvable by doing a revert (e.g. it’s an issue with the underlying CI infra), you should file a SEV using the [CI SEV issue template](https://github.com/pytorch/pytorch/issues/new?assignees&labels=ci%3A+sev&template=ci-sev.md). 
+
+## Rules of thumb 
+
+* If it’s bad enough that people multiple people will ask us about it on any of our contributor communication channels
+* If it affects more than ~10% of users and might last over 30 minutes, it’s a SEV
+* If we’re expecting people to force push to work around it, it’s a SEV
+
+If the above still leaves it ambiguous, we default to considering it a SEV.
+
+If the sev is severe enough (generally defined as "the CI is likely to miss legitimate failures", the template includes instructions on how to make it *merge blocking* to prevent contributors from accidentally merging broken code.
+
+# During a SEV
+[HUD](http://hud.pytorch.org) and the Dr. CI comment at the top of the PR will both explain that there's a SEV ongoing and link to the active sev.
+
+If the SEV is marked as merge blocking, @pytorchbot will default to preventing PRs from being merged