Update WCF Infrastructure documentation.

[StephenBonikowsky]

• https://github.com/dotnet/wcf/blob/master/Documentation/README.md

• https://github.com/dotnet/wcf/blob/master/Documentation/developer-guide.md#building-the-repository

• https://github.com/dotnet/wcf/blob/master/Documentation/issue-guide.md

• https://github.com/dotnet/wcf/blob/master/Documentation/manualtest-guide.md

• https://github.com/dotnet/wcf/blob/master/Documentation/scenario-test-guide.md

• https://github.com/dotnet/wcf/blob/master/Documentation/using-prerelease-wcf-packages.md

• https://github.com/dotnet/wcf/blob/master/Documentation/building/cross-platform-testing.md

• https://github.com/dotnet/wcf/blob/master/Documentation/building/unix-instructions.md