dx: better guide potential contributors on dealing with 'path-too-long'-errors on windows

[earloc]

@davidwengier , @ryzngard this is a followup issue on our recent discussion, which popped up during work on #11124

steps to reproduce:

• Checkout a fresh copy of this repo on a windows machine w/o enabled support for long-paths
• run the ./build.cmd
• observe errors occuring, such as:

error MSB4248: Cannot expand metadata in expression "$([MSBuild]::ValueOrDefault('%(FullPath)','').StartsWith($([MSBuild]::EnsureTrailingSlash($(MSBuildProjectDirectory)))))".
The item metadata "%(FullPath)" cannot be applied to the path "TestFiles\IntegrationTests\ComponentDesignTimeCodeGenerationTest\GenericComponent_GenericEventCallbackWithGenericTypeParameter_NestedTypeInference\TestComponent.mappings.txt". 
Path: {{PATH-TO-REPOSITORY}}\src\Compiler\Microsoft.AspNetCore.Razor.Language\test\TestFiles\IntegrationTests\ComponentDesignTimeCodeGenerationTest\GenericComponent_GenericEventCallbackWithGenericTypeParameter_NestedTypeInference\TestComponent.mappings.txt exceeds the OS max path limit.
The fully qualified file name must be less than 260 characters. [{{PATH-TO-REPOSITORY}}\src\Compiler\Microsoft.AspNetCore.Razor.Language\test\Microsoft.AspNetCore.Razor.Language.Test.csproj]

expected
Something telling me that I need to enable long path support, or other directions that I should be aware of when starting to contribute to this repo.

actual
Only errors occuring.