Draft explainer for Text API

Change-Id: I35ef67417fdfb5aa17dbeee92631d72294e9c29c
No-Try: true
Docs-Preview: https://skia.org/?cl=413837
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413837
Reviewed-by: Mike Reed <reed@google.com>
3 files changed