Add documentation on testing frontmatter (#810)

internal/readme.md

@@ -1,3 +1,20 @@
-### `Internal/`
+## `Internal/`
 
 This is internal repository and documentation management code. It does not contain examples.
+
+### Frontmatter
+
+Examples have frontmatter that control testing and deployment behavior. Fields:
+
+- `deploy`: If `true`, the example is deployed to the website with `modal deploy`. If `false`, it is not. Default is `false`.
+- `cmd`: The command to run the example for testing. Default is `["modal", "run", "<filename>"]`.
+- `args`: Arguments to pass to the command. Default is `[]`.
+- `lambda-test`: If `true`, the example is tested with the cli command provided in `cmd`. If `false`, it is not. Default is `true`.
+- `runtimes`: Control which runtimes the example is executed on in synthetic monitoring. Default is `["runc", "gvisor"]`.
+
+Example for a web app. Note that here we `modal serve` in the test so as to not deploy to prod when testing.
+
+```yaml
+deploy: true
+cmd: ["modal", "serve", "10_integrations/pushgateway.py"]
+```