Add CodeRush directory to .gitignore for C#

This is just a temporary directory for CodeRush, and should never
contain anything worth checking in. (Like the .vs directory.)
diff --git a/csharp/.gitignore b/csharp/.gitignore
index d0d7ae0..62c9638 100644
--- a/csharp/.gitignore
+++ b/csharp/.gitignore
@@ -18,6 +18,7 @@
 # 	Untracked files
 #
 .vs
+.cr
 *.user
 *.suo
 *.nupkg