Add R/rstats templates

[psolymos]

I created R/rstats templates using different HTTP frameworks and base images. I've decided to add the following 2 to the templates.json file of the store repository for the following reason:

• rstats-minimal: vanilla R + classic watchdog on Alpine base image, should be suitable as the smallest build for text/JSON based functions not needing additional R functionality
• rstats-base-plumber: R with plumber package installed + of-watchdog on Debian base image, this is the most versatile, plumber is mature with great community support and easy to add serializers/parsers through code annotation.

The https://github.com/analythium/openfaas-rstats-templates repository has other templates that might not be all suitable for mainstream consumption. The repo also has detailed documentation and an increasing number of examples.

R templates was discussed previously on Slack and offline, thanks for the great feedback.

I opened the related #114 request previously.

[derek]

Thank you for your contribution. I've just checked and your commit doesn't appear to be signed-off. That's something we need before your Pull Request can be merged. Please see our contributing guide.
Tip: if you only have one commit so far then run: git commit --amend --signoff and then git push --force.

[psolymos]

No-dco. Will try again.