[WIP] Add warning for ref to netstandard >= 1.5 from netfx without built-in support

[nguerrera]

I'm handing this off to a teammate while I'm away.

Still TODO:

• Confirm final error message text with PMs
• Make sure fwlink points to valid content
• Tests
  • The existing tests of the extensions adding support files probably just need to be updated to also check for the correct warning or lack thereof. The matrix is the same as the cases where we do/do not add support files:
  • .NETFramework < 4.7.2 -> .NETStandard >= 1.5 --> warning
  • .NETFramework any -> .NETStandard < 1.5 --> no warning
  • .NETFramework >= 4.7.2 -> .NETStandard any --> no warning
  • Warning can be suppressed

[nguerrera]

NOTE: Error code may still change as this will conflict with #2282

[nguerrera]

Another TODO item: suppression mechanism.

It should work to do <MSBuildWarningsAsMessages>NETSDK1066</MSBuildWarningsAsMessages> in .csproj. See dotnet/msbuild#1928

I think we should leverage this moving forward now that our errors have codes and stop adding individual booleans for everything.

But this needs testing.

The doc at the fwlink should cover how to suppress if things are working fine for you and you haven't been impacted by the known issues.

[dsplaisted]

Suggest:

This project uses a library that targets .NET Standard 1.5 or higher and targets a version of .NET Framework that does not have built-in support for that version of .NET Standard. Visit https://go.microsoft.com/fwlink/?linkid=874511 for a set of known issues. Consider retargeting to .NET Framework 4.7.2.

I'd also suggest creating an aka.ms link instead of a fwlink, that way we can choose a name that's easier to type in, for example http://aka.ms/net-standard-known-issues.

@mairaw Can you review this for style?

[mairaw]

Your suggestion LGTM @dsplaisted! Only nit per Microsoft style guide is the use of contractions
does not -> doesn't

[wli3]

Continue at #2330