Include Github link to OpenApi Tool Readme

My suggestion:, add an example of how to configure the csproj node setting to 'on build' generate an openapi file that SwaggerUI could use, NSwag could use to generate a 'connected client'.

Also, the github repo for Microsoft.dotnet-openapi is the 'dotnet aspnetcore' repo, it is quite big, so to assist the reader in finding an example and further guidance on configuring the entry in the csproj file, add a link to:

[https://github.com/dotnet/aspnetcore/blob/main/src/Tools/Microsoft.dotnet-openapi/README.md](https://github.com/dotnet/aspnetcore/blob/main/src/Tools/Microsoft.dotnet-openapi/README.md)

The contents seem to have been copy and pasted in this doc page, are they auto updated when the readme is updated?

[NSwag generates a client onBuild](https://github.com/RicoSuter/NSwag/wiki/NSwag.MSBuild) by Rico Suter

```

```

And example of ```.csproj``` [connected client using the generated .json](https://devblogs.microsoft.com/aspnet/generating-http-api-clients-using-visual-studio-connected-services/) by Jon Galoway:

```

OpenAPIs\ContosoOnlineOrders.Api.json

```

I have not found a blog post (not even Brady Gaster's: https://devblogs.microsoft.com/aspnet/creating-discoverable-http-apis-with-asp-net-core-5-web-api/?WT.mc_id=dotnet-13135-bradyg), so I'd really appreciate a complete example of using the openapi tool.

The new 'Microsoft.dotnet-openapi' example could also be used to improve the readme doc :-)

---
#### Document details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 653e14ef-227e-a165-0360-2f59259e16c6
* Version Independent ID: e4685d05-e874-76a3-9780-66b864eca6c1
* Content: [Develop ASP.NET Core apps using OpenAPI](https://docs.microsoft.com/en-au/aspnet/core/web-api/microsoft.dotnet-openapi?WT.mc_id=beginwebapis-c9-cephilli&view=aspnetcore-5.0)
* Content Source: [aspnetcore/web-api/Microsoft.dotnet-openapi.md](https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/web-api/Microsoft.dotnet-openapi.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-webapi**
* GitHub Login: @ryanbrandenburg
* Microsoft Alias: **rybrande**