add migration tooling docs

Author: bmorelli25

docs/migration/guide/tooling.md

@@ -2,4 +2,25 @@
 
 Use the [adoc-to-md](https://github.com/elastic/adoc-to-md) conversion tool to migrate content sets from Asciidoc syntax to docs-builder syntax. Instructions to use the tool are in the readme file.
 
+
+## Post-migration tooling
+
+After migrating content from asciidoc to md, there is cleanup work to do. @colleen has created a script to handle this process for us. The script:
+
+* Moves files to their new home in the new IA
+* Nests content at a pre-selected depth
+    * shortens directory and file names
+* Adds frontmatter mapping files to their asciidoc equivalent
+
+### File/Dir mappings
+
+* [`shortened-slugs.js`](https://github.com/elastic/docs-helpers/blob/post-migration-sort/post-migration-sort/input/field-mapping/shortened-slugs.js)
+* [`word-replacement.js`](https://github.com/elastic/docs-helpers/blob/post-migration-sort/post-migration-sort/input/field-mapping/word-replacement.js)
+
+## Post-migration manual work
+
+* Being tracked in https://github.com/elastic/docs-builder/issues/261
+
+## What's next?
+
 After running the migration tool, you can move and manipulate files while viewing a live preview of the content with docs-builder.