Updated docs on configuration and required env vars. (#285)

* Updated docs on configuration and required env vars.

* minor

examples/inputs/README.md

@@ -0,0 +1,15 @@
+# Tutorial 1: Go Function w/ Input (3 minutes)
+
+Example for putting required vars into the func.yaml file.
+
+Try running the following:
+
+```sh
+fn run
+# > ERROR: required env var SECRET_1 not found, please set either set it in your environment or pass in `-e SECRET_1=X` flag.
+SECRET_1=YOOO fn run
+# > info: optional env var SECRET_2 not found.
+# > {"SECRET_1":"YOOO","SECRET_2":"","message":"Hello World"}
+SECRET_1=YOOO SECRET_2=DAWG fn run
+# > {"SECRET_1":"YOOO","SECRET_2":"DAWG","message":"Hello World"}
+```