Update ADOPTERS.md
Update ADOPTERS.md

Include Ratehub in the list of OpenFaaS adopters.

Signed-off-by: Chris Richard <bac06ff1e952289f36f18637547c377fc90c7e2a@ratehub.ca>

rgee0/faas,alexellis/faas,rgee0/faas,alexellis/faas,rgee0/faas,openfaas/faas,rgee0/faas,openfaas/faas,rgee0/faas,rgee0/faas,rgee0/faas,openfaas/faas,alexellis/faas,alexellis/faas,alexellis/faas,alexellis/faas,rgee0/faas,rgee0/faas,alexellis/faas,alexellis/faas,alexellis/faas,rgee0/faas