Closes #4775 - Update README with Severity Triage process

[liuche]

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Updating README to explain quick severity triage process.

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

After merge

• Milestone: Make sure issues finished by this pull request are added to the milestone of the version currently in development.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@905c785). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #6311   +/-   ##
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  Coverage          ?   14.49%           
  Complexity        ?      323           
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  Files             ?      272           
  Lines             ?    11039           
  Branches          ?     1593           
=========================================
  Hits              ?     1600           
  Misses            ?     9311           
  Partials          ?      128

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[sblatz]

Looks good, thank you for adding this 😄