README: Slack workspace self-signup (#1192)
README: Slack workspace self-signup (#1192)

Signed-off-by: Simran Spiller <1371eb184de07f6be27e96421dea28de29b48198@gmail.com>
AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO