Update README.md "Known issues" section
Update README.md "Known issues" section

This seems like a better description of the actual contents of the section.  Expanded contents to mention the need for fully automated reporting and for crypto key storage.  Also reshuffled the list a bit to put the things likely to be top concerns first.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136471298

google/nomulus,google/nomulus,google/nomulus,google/nomulus,google/nomulus,google/nomulus