[Doc, readme.md] Debugging section in readme.md needs to be updated

[vlada-shubina]

https://github.com/dotnet/templating#debugging

• dn3buildmode-debug.cmd script is not available in repository
• it is not clear what 'setup' script means
• attaching to the process works even without executing scripts mentioned

https://github.com/dotnet/templating#template-nuget-packages

• mentions that installing templates from the packages requires Windows, however installing templates from artifacts works both on Windows and MacOS