README.md: Adapt to the correct release versioning scheme
README.md: Adapt to the correct release versioning scheme

Our current release versioning scheme needs the revision to be added
to a meta-resin version using the plus character (+) instead of the
dot character (.)

Signed-off-by: Florin Sarbu <3a837a37a2ac9072a088641ec330e0e5d5b86f4e@resin.io>

resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin