Injecter Extension documentation does not document environment variables

[SamStephens]

In the Injecter Extension section of README.md, we are told that the extension "adds the necessary environment variables to the tasks that are part of the service". However it does not document the naming used for the environment variables; the only way to discover what they are called is either to add the extension to your service and then see what variables are available, or to read the code.