Add doc link for installing Xcode

Change-Id: Ib1973d57fab3c7a509955451c3d65bf1828991e1
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/551889
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>