Bug 875562 - Part 5: Document crash events for initial implementation; r=bsmedberg

The initial implementation of this feature will write 3 distinct event
types and will only count occurrences of each. Future iterations of the
feature will capture more metadata, such as submission state and extra
crash annotations. These will not be included in this patch set.

DONTBUILD (NPOTB)

--HG--
extra : rebase_source : a1be4ceb6e06f2153d0927e50090c5f352b741d6