Add VS2017 optional component dependency details to the C# ReadMe for #4119

[dchennells]

Add mention to the "Build" section of the ReadMe of the requirement of installing an optional component to VS2017, as discussed with @jskeet in #4119.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

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[jskeet]

I'm fine with this - assigned @anandolee as protobuf team member.

[dchennells]

I signed the Contributor License Agreement (CLA).

[googlebot]

CLAs look good, thanks!