docs: add func.yaml field documentation

docs/guides/func_yaml.md

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+# Project Configuration with `func.yaml`
+
+The `func.yaml` file contains configuration information for your function
+project. Generally, these values are used when you execute a `func` CLI
+command. For example, when `func build` is run, the CLI uses the value for
+the `builder` field. In many cases, these values may be overridden by
+command line flags or environment variables. For more information about
+overriding these values, consult the [Commands](command.md) document.
+
+Many of the fields are generated for you when you create, build and deploy
+your function. However there are a few that you may use to tweak things
+such as the function name, and the image name.
+
+## Fields
+
+The following fields are used in `func.yaml`.
+
+### `builder`
+
+Specifies the buildpack builder image to use when building the function.
+In most cases, this value should not be changed.
+
+### `builderMap`
+
+Some function runtimes may be built in multiple ways. For example, a Quarkus
+function may be built for the JVM, or as a native binary. The `builderMap`
+field will contain all of the available builders for a given runtime. Although
+it's typically unnecessary to modify the `builder` field, using values from
+`builderMap` is OK.
+
+### `env`
+
+The `env` field allows you to set environment variables that will be
+available to your function at runtime. For example, to set a `MODE` environment
+variable to `debug` when the function is deployed, your `func.yaml` file
+may look like this.
+
+```yaml
+env:
+  MODE: debug
+  API_KEY: {{ env.API_KEY }}
+```
+
+### `image`
+
+This is the image name for your function after it has been built. This field
+may be modified and `func` will create your image with the new name the next
+time you run `kn func build` or `kn func deploy`.
+
+### `imageDigest`
+
+This is the `sha256` hash of the image manifest when it is deployed. This value
+should not be modified.
+
+### `name`
+
+The name of your function. This value will be used as the name for your service
+when it is deployed. This value may be changed to rename the function on
+subsequent deployments.
+
+### `namespace`
+
+The Kubernetes namespace where your function will be deployed.
+
+### `runtime`
+
+The language runtime for your function. For example `python`.
+
+### `trigger`
+
+The invocation event that triggers your function. Possible values are `http`
+for plain HTTP requests, and `events` for CloudEvent triggered functions.
+
+
+## Environment Variables
+
+Any of the fields in `func.yaml` may contain a reference to an environment
+variable available in the local environment. For example, if I would like
+to avoid storing sensitive information such as an API key in my function
+configuration, I may have this value set from the local environment. To do
+this, prefix the local environment variable with `{{` and `}}` and prefix
+the name with `env.`. For example:
+
+```yaml
+env:
+  API_KEY: {{ env.API_KEY }}
+```