updated SkSL README

Bug: skia:
Change-Id: Iac2f0451ab86a9fa17e119a562c1305b31aa1069
Reviewed-on: https://skia-review.googlesource.com/102144
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed