Reduce duplication of runtime dependencies locally + Update README

[MaxInertia]

Currently the runtime dependencies appear in the top-level ./package.json and ./dependencies/nodejs/package.json. The former for IDE's to index for code completion and the latter for specifying what's uploaded as a lambda layer. It would be nice to remove this duplication. Symbolic links might be useable to avoid duplicates of the runtime dependencies in both node_modules. That would still leave duplicates in the package.json and package.lock, whose versions could potentially go out of sync. That's rubbish.

• solve the duplication issue above
• update the README with the procedure for adding new runtime dependencies