Update basic Fuchsia building docs

Bug: None
Change-Id: I721c75e75ae0ec95667749f673578221747cbbea
Reviewed-on: https://chromium-review.googlesource.com/1244156
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594085}
3 files changed