designdoc: Add description of user data sources.
designdoc: Add description of user data sources.

Bug: crashpad:167
Change-Id: I8c5032d6710ea828e354103743d34772fd083958
Reviewed-on: https://chromium-review.googlesource.com/472947
Commit-Queue: Sigurður Ásgeirsson <592632e08e8950241f83f5341243136380567d51@chromium.org>
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

atom/crashpad,chromium/crashpad,chromium/crashpad,atom/crashpad,chromium/crashpad,atom/crashpad