Add comment explaining the derivation of our Mitchell coefficients

BUG=skia:

Change-Id: I8866df425ee9837e75f0b2f76777f7e5d68fb21d
Reviewed-on: https://skia-review.googlesource.com/5624
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed