SkPDF/docs: note that Sfntly subsetter is deprecated

Change-Id: I09ecee95653212268926d82739c0df3866d3a8d7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253380
Auto-Submit: Hal Canary <halcanary@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
1 file changed