Auto merge of #5383 - dwijnand:contributing-custom-build, r=alexcrichton
authorbors <bors@rust-lang.org>
Wed, 18 Apr 2018 02:28:35 +0000 (02:28 +0000)
committerbors <bors@rust-lang.org>
Wed, 18 Apr 2018 02:28:35 +0000 (02:28 +0000)
add build-from-source info to Tests architecture docs

it's mentioned in the README, but it's worth mentioning again in the
Tests section of the architecture doc.


Trivial merge