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/triggers.md

@@ -0,0 +1,17 @@
+# Trigger
+
+A Trigger represents an entry point for function invocations. Each type of Trigger requires specific configuration. They should be defined within the func.yaml file, as specified in [FUNC YAML](func-file.md).
+
+## Types
+
+### http
+
+Configures a http endpoint for function invocation.
+
+```
+name: triggerOne
+type: http
+source: /trigger-path
+```
+
+This will cause the system to route requests arriving at the fn service at `/trigger-path` to the function specified in the fn.yaml file.