Add helloworld example

This is the first thing that someone new to Dagger tried to do.

I wanted to capture it in an example so that we can reference in the future.

I am sure that we can improve on this small step in a direction that I
have seen a new member take.

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

pkg/universe.dagger.io/examples/helloworld/main.cue

@@ -0,0 +1,24 @@
+// dagger do hello --log-format=plain
+//
+// 9:06AM INF actions._alpine | computing
+// 9:06AM INF actions._alpine | completed    duration=1s
+// 9:06AM INF actions.hello | computing
+// 9:06AM INF actions.hello | #3 0.073 hello, world!
+// 9:06AM INF actions.hello | completed    duration=100ms
+package main
+
+import (
+	"dagger.io/dagger"
+)
+
+dagger.#Plan & {
+	actions: {
+		_alpine: dagger.#Pull & {source: "alpine:3"}
+		// Hello world
+		hello: dagger.#Exec & {
+			input: _alpine.output
+			args: ["echo", "hello, world!"]
+			always: true
+		}
+	}
+}