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

* Updated docs on configuration and required env vars.

* minor

docs/developers/configs.md

@@ -0,0 +1,20 @@
+# Function Config Vars
+
+There are three ways to get configuration variables into a function, all of which will be available as environment variables.
+These are ordered in order of preference, the later ones overriding the previous ones.
+
+## 1. Application level configuration
+
+```sh
+fn apps config set myapp LOG_LEVEL debug
+```
+
+## 2. Function configuration from func.yaml
+
+See [Function file](../function-file.md) for more info.
+
+## 3. Route level configuration
+
+```sh
+fn routes config set myapp hello2 LOG_LEVEL info
+```