Update README

[gsnedders]

The README.md claims:

Kuma is the platform that powers MDN (developer.mozilla.org)

This isn't true. The README should be accurate and describe the current status of Kuma and any usage of it (I presume there is usage given some people appear to still be contributing?), and probably should link to Yari too.

People asking about how to do things in Kuma for the live MDN sites happens every so often in https://matrix.to/#/#mdn:mozilla.org, so there's definitely some degree of people being confused by this.

[escattone]

@gsnedders Thanks, yes, we definitely need to update the README. Kuma is still used, but now instead of being the "platform that powers MDN", it's the platform that provides MDN's API services.

[Uzume]

...so that can be easily changed to specify it as the "platform that powers MDN API services" and of course mention the history of powering all of the MDN and how the main content is now "powered" by Yari (perhaps even with some comments about reimplementing KumaScript there, etc.),

[caugner]

Kuma has been replaced by Rumba in the mean-time, and the README is now updated accordingly.