Add build instructions for Linux and MacOS BUG=angleproject:1368 Change-Id: I2a0852399cdf04192d7e1d11b33b2103aa173354 Reviewed-on: https://chromium-review.googlesource.com/344003 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>