Add OpenCLI integration to Spectre.Console.Cli

docs/input/cli/opencli.md

@@ -0,0 +1,22 @@
+Title: OpenCLI Integration
+Order: 15
+Description: OpenCLI integration
+Highlights:
+ - Generate OpenCLI descriptions
+---
+
+From version `0.52.0` and above, you will be able to generate [OpenCLI](https://opencli.org)
+descriptions from your `Spectre.Console.Cli` applications.
+
+Simply add the `--help-dump-opencli` option to your application, and an 
+OpenCLI description will be written to stdout.
+
+```shell
+$ ./myapp --help-dump-opencli
+```
+
+If you want to save it to disk, pipe it to a file.
+
+```shell
+$ ./myapp --help-dump-opencli > myapp.openapi.json
+```