Updated README

README.md

@@ -7,28 +7,28 @@ Note this plugin should work equally well despite any personal preferences for e
 
 ## Example
 
-To read the value from an environment variable named `MY_TOKEN` when the plugin executes, use the `env:` prefix.
+To read the value from an environment variable named `MY_TOKEN` when the plugin executes, use the `env` fiels.
 
 ```yml
 steps:
   - command: yarn install
     plugins:
       seek-oss/private-npm#v1.1.1:
-        token: "env:MY_TOKEN"
+        env: "MY_TOKEN"
 ```
 
-To read the value from a file named `my_token_file`, use the `file:` prefix.
+To read the value from a file named `my_token_file`, use the `file` field.
 
 ```yml
 steps:
   - command: yarn install
     plugins:
       seek-oss/private-npm#v1.1.1:
-        token: "file:my_token_file"
+        file: "my_token_file"
 ```
 
 Alternatively you can read the token directly from any value exposed toxs your `pipeline.yml` file.  However, this 
-approach is discoraged in favour of using with the `env:` or `file:` prefix.  This functionality remains in the interest
+approach is discoraged in favour of using with the `env` or `file` fields.  This functionality remains in the interest
  of backwards compatibility.
 
 ```yml
@@ -47,24 +47,31 @@ steps:
   - command: yarn install
     plugins:
       seek-oss/private-npm#v1.1.1:
-        token: ${MY_TOKEN}
+        env: "MY_TOKEN"
         registry: //myprivatenpm.com/
 ```
 
 ## Configuration
 
-### `token` (required)
-The value of the NPM token.  Using the prefix `env:` of `file:` changes the behaviour of how the token is read.  By 
-omitting the prefix, the value will be read from a variable which is available to the Buildkite YAML parsing context.  
-This value is interpolated when the YAML configuration is parsed by the Buildgent agent and provided to the plugin "as 
-is".
+> **NOTE** Even thought `env`, `file` and `token` are described as optional, _at least one must be set_ or the plugin 
+> will fail.
+
+### `env` (optional)
 
-Using the `env:` prefix delays reading the value from the agent environment until the plugin executes.
+The value of the NPM token will be read from the agent environment when the plugin executes.  This is useful in working
+around cases where eager binding of variables in `pipeline.yml` means some variables are not present in the 
+environment when the configuration file is parsed.
 
-Using the `file:` prefix will attempt to read the value from a file.
+### `file` (optional)
 
-The prefix variants are recommended if you are using a secrets manager or some other kind of late variable binding 
-mechanism to configure your agent's environment.  Please see the examples for useage.
+The value of the NPM token will be read from a file on the agent when the plugin executes.  This is useful when working
+with secret that are created as files on the filesystem when a build is initiated.
+
+### `token` (optional)
+
+The value of the NPM token will be read from a variable which is available to the Buildkite YAML parsing context.  
+This value is interpolated when the YAML configuration is parsed by the Buildgent agent and provided to the plugin "as 
+is".
 
 Example: `${MY_TOKEN}`
 > **NOTE:** Don't put your tokens into source control.  Don't use web interfaces you don't control to inject them into