Add Batch Composite plugin page

Co-authored-by: Curtis Rueden <ctrueden@wisc.edu>

Author: kfish610

_pages/plugins/batch-composite.md

@@ -0,0 +1,11 @@
+---
+title: Batch Composite
+categories: [Automation, Color Processing, Import-Export, Stacks]
+---
+
+Batch Composite is a small plugin that runs the Composite operation on a folder
+of openable files. It supports Bio-Formats, recursion, and color customization.
+
+It can be added using the update site:
+
+{% include link-banner url="https://sites.imagej.net/BatchComposite/" %}