Update Java invoker readme

Signed-off-by: Andrew Su <[email protected]>

buildpacks/java/README.md

@@ -4,18 +4,28 @@ The Java Function Buildpack is a Cloud Native Buildpack that provides a Spring B
 
 ## Behaviour
 This buildpack will participate if any of the following conditions are met:
-* A file with the name `func.yaml` is detected
+* One of the configuration environment variables are explicitly set.
+* A file with the name `func.yaml` is detected.
 
 The buildpack will do the following if detection passed:
 * Request for a JRE to be installed
 * Contributes the Spring Boot application to a layer marked `launch` with the layer's path prepended to `$CLASSPATH`
 * Contributes environment variables defined in `func.yaml` to the `launch` layer
+* Contributes environment variables to configure Spring Boot if any configuration variables are defined. (Overrides anything from `func.yaml`)
+
+## Configuration
+
+| Environment Variable | Description |
+|----------------------|-------------|
+| `$BP_FUNCTION` | Configure the composition of functions. Defaults to empty (load all functions). |
+| `$BP_FUNCTION_PACKAGES`    | Configure the package to search for functions.  Defaults to `functions`. |
+
 
 ## Getting started
 To get started you'll need to create a directory where your function will be defined.
 
 From within this directory we require a few files to properly detect this as a Java function:
-* `func.yaml`: We use this to configure the runtime environment variables. See the [Knative Func CLI docs](https://github.com/knative-sandbox/kn-plugin-func/blob/main/docs/guides/func_yaml.md) for more details.
+* `func.yaml` (optional): We use this to configure the runtime environment variables. See the [Knative Func CLI docs](https://github.com/knative-sandbox/kn-plugin-func/blob/main/docs/guides/func_yaml.md) for more details.
 * `pom.xml` or `build.gradle`: These are used by the other Java buildpacks to compile your function.
 * Java package in folder `src/main/java/functions`: This is the default location your function will be detected. If you do choose to use another package to store your functions, you will need to [set a new search location](#TODO).