Add a section on readme about running the functions outside a container with Node.js

[helio-frota]

There is a gap in the docs between the ./func create and ./func build steps.

Lance shared the following information in our Slack channel that helps with this:

Some of the Knative function runtimes work out of the box without requiring a build. 
These are the interpreted languages right now - Node.js, TypeScript and Python.

running a function outside of a container for testing and 
general ease of use in the “inner loop” development cycle.

[helio-frota]

a2d8c56