Add a THANKS file

THANKS.md

+# The dav1d project and VideoLAN association would like to thank
+
+## AOM
+The Alliance for Open Media (AOM) for funding this project.
+
+## Companies
+* Two Orioles LLC, for important coding effort
+* VideoLabs SAS
+
+## Projects
+* VideoLAN
+* FFmpeg
+
+## Individual
+
+And all the dav1d Authors (git shortlog -sn)