Merge pull request #420 from girder/migration-note

Add a note about migration from Girder 2.

README.rst

@@ -68,6 +68,20 @@ Many tile sources have complex prerequisites.  These can be installed directly u
     ``pip install -e large-image[memcached] -r requirements-dev.txt --find-links https://girder.github.io/large_image_wheels``
 
 
+Migration from Girder 2 to Girder 3
+===================================
+
+If you are migrating a Girder 2 instance with Large Image to Girder 3, you need to do a one time database update.  Specifically, one of the tile sources' internal name changed.
+
+Access the Girder Mongo database.  The command for this in a simple installation is::
+  
+    mongo girder
+
+Update the tile source name by issuing the Mongo command::
+
+    db.item.updateMany({"largeImage.sourceName": "svs"}, {$set: {"largeImage.sourceName": "openslide"}})
+
+
 .. _Girder: https://github.com/girder/girder
 
 .. |build-status| image:: https://travis-ci.org/girder/large_image.svg?branch=master