add build clarification to getting started docs

Change-Id: I3872d290f969a552c2d6ad1d7f63739e68977806
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/896737
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>
1 file changed