Add rustfmt gitignore (#2108)

waynenilsen · shiftkey · commit 309aa7aba03d · 2016-10-03T14:46:39.000+11:00
diff --git a/Rust.gitignore b/Rust.gitignore
@@ -5,3 +5,6 @@
 # Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
 # More information here http://doc.crates.io/guide.html#cargotoml-vs-cargolock
 Cargo.lock
+
+# These are backup files generated by rustfmt
+**/*.rs.bk
