Plan for func.yaml file changes for triggers. fnproject/cli#324 (#1115)

The changes aim to be as minimal as possible:

* Remove no longer available options
* Add a schema version to the file for validation
* Add trigger list block

docs/developers/func-file.md

@@ -0,0 +1,69 @@
+# Func files
+
+Func files are used to assist the fn-project to help you when creating functions.
+
+An example of a func file:
+
+```yaml
+schema_version: 20180708
+name: fnproject/hello
+version: 0.0.1
+runtime: java
+build_image: x/y
+run_image: x/y
+cmd: com.example.fn.HelloFunction::handleRequest
+memory: 128
+timeout: 30
+config:
+  key: value
+  key2: value2
+  keyN: valueN
+build:
+- make
+- make test
+expects:
+  config:
+    - name: SECRET_1
+      required: true
+    - name: SECRET_2
+      required: false
+triggers:
+  - name: triggerOne
+    type: http
+    source: /trigger-path
+```
+`schema_version` represents the version of the specification for this file.
+
+`name` is the name and tag to which this function will be pushed to and the
+route updated to use it.
+
+`version` represents the current version of the function. When deploying, it is appended to the image as a tag.
+
+`runtime` represents programming language runtime, for example,
+'go', 'python', 'java', etc.  The runtime 'docker' will use the existing Dockerfile if one exists.
+
+`build_image` (optional) base Docker image to use for building your function. Default images used are the `dev` tagged images from the [dockers repo](https://github.com/fnproject/dockers).
+
+`run_image` (optional) base Docker image to use for running your function, part of a multi-stage build. Function will be built with `build_image` and run with `run_image`. Default images used from the [dockers repo](https://github.com/fnproject/dockers).
+
+`cmd` (optional) execution command for jvm based runtimes
+
+`entrypoint` (optional) excution entry point for native runtimes
+
+`memory` (optional) allows you to set a maximum memory threshold for this
+function. If this function exceeds this limit during execution, it is stopped
+and error message is logged. Default: `128`.
+
+`timeout` (optional) is the maximum time a function will be allowed to run. Default is 30 seconds.
+
+`config` (optional) is a set of configuration variables to be passed onto the function as environment variables.
+These configuration options shall override application configuration during functions execution. See [Configuration](configs.md)
+for more information.
+
+`expects` (optional) a list of config/env vars that are required to run this function. These vars will be used when running/testing locally,
+if found in your local environment. If these vars are not found, local testing will fail.
+
+`build` (optional) is an array of local shell calls which are used to help
+building the function. TODO: Deprecate this?
+
+`triggers` (optional) is an array of `trigger` entities that specific triggers for the function. See [Trigger](triggers.md).