Add Azure Pipelines CI badge, maybe deprecate others in future

[hsluoyz]

We know .NET is from Microsoft. Azure Pipelines is also from Microsoft. So using Azure Pipelines as our CI will show strong credit that our code is well tested.

Moreover, our evaluator is also using it: https://github.com/davideicardi/DynamicExpresso

So we should add a CI badge for Azure Pipelines. Maybe other CI badges will be deprecated in future.

[hsluoyz]

@huazhikui @nodece @xcaptain @sagilio can anyone take this issue?

[sagilio]

I can do it, but I need permission to visit the Azure DevOps. You can use Sagilio@outlook.com to invite me at Azure DevOps.

[huazhikui]

one month ago, I has added Microsoft. Azure Pipelines ，but i have not add the CI badge @sagilio @hsluoyz
https://dev.azure.com/Casbin/Casbin.NET/_build?view=pipelines

[hsluoyz]

@huazhikui great news to hear!

@sagilio can you make a PR to add the badge for: https://dev.azure.com/Casbin/Casbin.NET/_build?view=pipelines ?

[hsluoyz]

@sagilio you can also remove Appveyor totally in the new PR.

[sagilio]

@sagilio you can also remove Appveyor totally in the new PR.

We need Appveyor to provide Coverage Status now.

[hsluoyz]

OK. Then we only add the Azure Pipelines badge for now.

[hsluoyz]

one month ago, I has added Microsoft. Azure Pipelines ，but i have not add the CI badge @sagilio @hsluoyz
https://dev.azure.com/Casbin/Casbin.NET/_build?view=pipelines

@huazhikui can you add my Microsoft account: hsluoyz@163.com to the admin of the Casbin org in DevOps? https://dev.azure.com/Casbin/