Update .gitignore (dotnet#87)

Author: Rick-Anderson

.gitignore

@@ -3,6 +3,9 @@
 ##
 ## Get latest from https://github.com/github/gitignore/blob/master/VisualStudio.gitignore
 
+# Use git add -f to force override .sln when required. Not needed in most cases.
+# git add -f myProj.sln
+
 Properties/
 *.sln