adding gravity example

Author: eyberg

gravity/README.md

@@ -0,0 +1,10 @@
+Gravity Unikernel Hello World
+==================
+
+```sh
+gravity -c hi.gravity -o hi.json
+```
+
+```sh
+$ ops run gravity -c config.json
+```

gravity/config.json

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+{
+  "Files": ["hi.json"],
+  "Args": ["-x", "hi.json"]
+}

gravity/hi.gravity

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+func main() {
+  System.print("Hello World!")
+}