Update .gitignore to exclude Rider workspace files.
Update .gitignore to exclude Rider workspace files.

cdmihai/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,mono/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,Microsoft/msbuild,chipitsine/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,mono/msbuild,cdmihai/msbuild,Microsoft/msbuild,jeffkl/msbuild,rainersigwald/msbuild,chipitsine/msbuild,rainersigwald/msbuild,mono/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,chipitsine/msbuild,cdmihai/msbuild,mono/msbuild,chipitsine/msbuild,jeffkl/msbuild,Microsoft/msbuild