Added transition page to docs with python script.

Author: kpinnipa

docs/user/transition.rst

@@ -0,0 +1,63 @@
+Transition to 2.1.0
+===================
+
+The 2.1.0 update has made snippets globally accessible across notebooks by saving 
+them at the JupyterLab Settings API endpoint.
+
+One side effect of this change is that snippets created in the previous version of 
+this extension will not be supported. The /snippets folder will continue to be available
+if previously created in a project folder.
+
+
+Transferring Single Snippets
+----------------------------
+
+To add old snippets to the new snippet location, simply copy the JSON object and go to:
+Settings > Advanced Settings Editor > Code Snippet Manager
+
+In the User Preferences panel there will be an array of snippets existing globally
+(something like "snippets" : [...]) where the ... are the globally accessible
+snippets.
+
+At the end of the existing list, paste the copied JSON object, make sure the id value is
+sequential to what already is in the list. Save using the save icon in the top right corner.
+Snippet should appear in the snippets panel!
+
+
+Transferring Multiple Snippets
+------------------------------
+
+To help with converting entire /snippets folder worth of snippets we have developed a python
+script to help with the transition:
+
+.. code::
+
+    import os
+    import json
+    import glob
+
+    def extract_id(json):
+        try:
+            return json['id']
+        except KeyError:
+            return 0
+
+    snippets = []
+    counter = 0
+    for filepath in glob.glob(os.path.join('snippets', '*.json')):
+        with open(filepath) as f:
+            content = json.load(f)
+            content['id'] = counter
+            snippets.append(content)
+            counter+=1
+
+    snippets.sort(key=extract_id)
+    for snip in snippets:
+        if not('tags' in snip):
+            snip["tags"] = []
+        print(snip, end=",\n")
+
+
+This script will concatenate and print out all of the json objects in a /snippets folder
+in a project. After running the script, copy the output and paste into the User Preferences
+panel in settings.