Update doc/developing.md with linux details
Update doc/developing.md with linux details

This information is very helpful to someone new to Crashpad to be able
to get up and running quickly (i.e. what I would have needed).

Bug: crashpad:291
Change-Id: Ibc84a009dbd7c93dd098e658cbe895957c6dca16
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/1567847
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>
Commit-Queue: Peter Wen <53d52474b70a71c67f70feffe31c51d063609fa8@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad