Add working link to official repo for couchbase-lite-js

First off, this new framework seems awesome, I'm very excited to try it out!

I wanted to give a quick heads-up though that I noticed via [couchbase-lite-js's NPM README](https://www.npmjs.com/package/@couchbase/lite-js) that the repo at https://github.com/couchbaselabs/couchbase-lite-js is still private. Once it goes public, it'd be great to have a link to it in the docs (I found myself scouring the various Couchbase Github orgs looking for the repo before I stumbled into the NPM package).