Add Azure DevOps build

[Szer]

Travis is pretty slow. Azure is much more powerful tool to build stuff.

I've tested this on my Azure DevOps account and it builds perfectly with only this yaml:

pool:
  vmImage: 'vs2017-win2016'

steps:
- script: fake.cmd build
  displayName: 'Build'

Build results:
https://szer.visualstudio.com/Hopac/_build/results?buildId=21&view=logs

[haf]

Ok, I don't mind adding a link to your account in the README... What needs to be done to integrate this with the build and PRs?

[Szer]

@haf I'm not repo owner so I can't authorize webhooks from Hopac repo propagate to my DevOps account. I've managed to build fork of Hopac :)

It has to be one of the maintainer personal accounts with permissions for other maintainers.
In case of maintainer retirement, it could be easily switched to another Azure account because yaml lies in repo itself.

For example, all .Net core projects (including visualfsharp) are being built here

You could follow this guide