Update Skia build instructions in build.md

Adds a brief discussion of supported compiler versions, and how to pull
changes and rebuild.

Bug: NONE
Change-Id: Ibd26e623c1435953969f9e69b00612fdff601881
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/270296
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed