Add a note about dist-upgrade in FAQ.md
Add a note about dist-upgrade in FAQ.md

Former-commit-id: e0ef2f6d4e73f1ffd39ebde50475ad54cca54eb4 [formerly 0d7aaf87761f5f402746a07a10341d0ffbfbcd3f] [formerly 0d7aaf87761f5f402746a07a10341d0ffbfbcd3f [formerly 4dc55cfae2c22c13e03c1aad689642960beb3bc9]]
Former-commit-id: 8bf773ce1a8b5eaf809d9582c57ba86cf1d6a753
Former-commit-id: c8447e0c01d3e027438d58fad616f4b856452182
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine