mono: add documentation and tryout example

Add a try out example and documentation to run a dotnet complatible
`executable` natively on linux.

Signed-off-by: Aditya R <arajan@redhat.com>

docs/mono-example.md

@@ -0,0 +1,56 @@
+# mono windows dotnet handler
+* Make sure oci config contains handler for **mono** or image contains annotation **run.oci.handler=dotnet**.
+* Entrypoint must point to a valid **.exe** (windows .NET compatible executable).
+ ```json
+...
+"annotations": {
+  "run.oci.handler": "dotnet"
+},
+...
+```
+
+## Examples
+#### Compile and run `wasm` modules directly
+* Following example is using `mono` to compile a cross platform executable but you can also use visual studio or any other build tools on windows.
+* Add relevant function to `hello.cs` for this example we will be using a print.
+ ```c#
+  using System;
+  using System.Runtime.CompilerServices;
+
+  class MonoEmbed {
+	static int Main ()
+	{
+		System.Console.WriteLine("hello");
+		return 0;
+	}
+  }
+
+```
+* Compile a new `.exe` using `mcs -out:hello.exe hello.cs` if you have `mono` or you can `VisualStudio` or `dotnet build` as specified here: https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-build
+* Create relevant image and use your container manager. But for this example we will be running directly using crun and plub config manually.
+```console
+$ crun run container-with-mono
+hello
+```
+
+#### Running OCI `mono` compat images with buildah and podman
+* Compile your `.exe` module using instructions from step one.
+* Prepare a `Containerfile` with your `.exe`.
+ ```Containerfile
+ FROM scratch
+COPY hello.exe /
+CMD ["/hello.exe"]
+ ```
+* Build wasm image using buildah with annotation `run.oci.handler=dotnet`
+```console
+$ buildah build --annotation "run.oci.handler=dotnet" -t my-windows-executable .
+```
+* Make sure your podman points to oci runtime `crun` build with `mono` support.
+* Run image using podman
+```console
+$ podman run --userns=keep-id my-windows-executable:latest
+hello
+```
+
+#### Known-Issues
+* Crun-mono containers needs user namespace for containers so with podman use `--userns=auto` or `--userns=keep-id`.