Ignore the polygerrit plugins directory

Change-Id: Ib64ad953383fd289be844a37245ab917098446f6
diff --git a/polygerrit-ui/app/.gitignore b/polygerrit-ui/app/.gitignore
new file mode 100644
index 0000000..375a75d
--- /dev/null
+++ b/polygerrit-ui/app/.gitignore
@@ -0,0 +1 @@
+/plugins/