updated your guidelines for core node "inject"

[robskaar]

Hi, i'm developing an "overlay" for node-red that enables a webapp to show dashboards for interacting with flows and edit a flow in a react webapp project. The inject core node guide line don't mention a lot about the API for the inject node, so i thought i'd supply it. i really needed this information at this specific spot, and i had to look long to find it elsewhere, hope this helps out others who also wonder about the inject node API.

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: robskaar / name: Robert Bjørnstjerne Skaar (a40ac5374673dddba4b7e6be9a9360e0a79e3a0a)

[Steve-Mcl]

@knolleary ok to merge please?

[knolleary]

Thanks @robskaar !