Ignore solution files (dotnet#5101)

Author: gewarren

.gitignore

@@ -12,6 +12,10 @@ rebuild.cmd
 *.userosscache
 *.sln.docstates
 
+[Bb]in/
+[Oo]bj/
+*.sln
+
 # User-specific files (MonoDevelop/Xamarin Studio)
 *.userprefs
 
@@ -225,4 +229,4 @@ samples/framework/docker/MVCRandomAnswerGenerator/containerImage
 .DS_Store
 _dependentPackages/
 !/xml/System.IO.Log/
-!/xml/System.Net.Cache/
+!/xml/System.Net.Cache/