Add missing description image (#57)

merge_description.png seems to be missing on the website[1].

This is a copy of the file in the procesor documentation.

[1](https://streampipes.apache.org/docs/docs/pe/org.apache.streampipes.processors.filters.jvm.merge.html)
diff --git a/website/img/pipeline-elements/org.apache.streampipes.processors.filters.jvm.merge/merge_description.png b/website/img/pipeline-elements/org.apache.streampipes.processors.filters.jvm.merge/merge_description.png
new file mode 100644
index 0000000..a08c6b1
--- /dev/null
+++ b/website/img/pipeline-elements/org.apache.streampipes.processors.filters.jvm.merge/merge_description.png
Binary files differ