Update readme for map cleanup plugin

Signed-off-by: Robert D Anderson <[email protected]>

README.md

@@ -12,4 +12,7 @@ is an entirely new format. Based on [But what if I _want_ a whole new transform
 1. `org.metadita.html5zip`: Hopefully helpful plugin that demonstrates how to use the `temp.output.dir.name` parameter
 (added in DITA-OT 2.5). This allows plugins to easily build off of existing transform types like HTML5, while easily
 adding post-processing steps (handled inside the temp directory) and/or returning a zip instead of discreet files.
-Described in the post [Making zippy HTML (or other zippy output)](http://metadita.org/toolkit/zippy.html)
+Described in the post [Making zippy HTML (or other zippy output)](http://metadita.org/toolkit/zippy.html)
+1. `org.metadita.cleanupmap`: Quick and easy plugin that works around an issue in DITA-OT processing of
+bookmaps that happen to use `<part>` to pull in maps. Based on a long thread in the [DITA-OT Slack channel](https://dita-ot.slack.com/archives/C03SENFUQ/p1536160123000100)
+and may warrant a blog post ... or maybe the readme will be enough?