cmd/config: Add examples and tutorials for config functions

- Add examples under `functions`
- Add built-in tutorial for functions

functions/examples/injection-tshirt-sizes/image/Dockerfile

@@ -0,0 +1,15 @@
+# Copyright 2019 The Kubernetes Authors.
+# SPDX-License-Identifier: Apache-2.0
+
+FROM golang:1.13-stretch
+ENV CGO_ENABLED=0
+WORKDIR /go/src/
+COPY go.mod .
+COPY go.sum .
+RUN go mod download
+COPY main.go .
+RUN go build -v -o /usr/local/bin/config-function ./
+
+FROM alpine:latest
+COPY --from=0 /usr/local/bin/config-function /usr/local/bin/config-function
+CMD ["config-function"]