Add out/ to .gitignore.

Change-Id: I518501f08fe7ad37b6e104c76bd4e557807deba6
diff --git a/.gitignore b/.gitignore
index 1442829..7ba2297 100644
--- a/.gitignore
+++ b/.gitignore
@@ -44,3 +44,6 @@
 # Yarn
 node_modules
 web_modules
+
+# Unknown
+out/