Revise documentation on adding EGL extensions. Provided more links and steps per earlier advice from geofflang@. Bug: angleproject:4842 No-Try: true Change-Id: I9c9361b4b71a63fd36f79a87d45e0f4b04c4140c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2297528 Commit-Queue: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>