Release candidate for OpenSubdiv v3.6.1

[David G Yu]

We've tagged a release candidate for the next release of OpenSubdiv:
  https://github.com/PixarAnimationStudios/OpenSubdiv/releases/tag/v3_6_1_RC1

More detailed release notes here:

  Release 3.6.1

We intend to tag the release next week. Please post any questions or feedback you may have here or file issues on our GitHub repository.

Thanks!
-David