Still more READMEs

packages/application-extension/README.md

@@ -1 +1,3 @@
 # @jupyterlab/application-extension
+
+An extension for JupyterLab which provides some top-level commands and services for the [@jupyterlab/application](../application) package.

packages/application/README.md

@@ -1 +1,4 @@
 # @jupyterlab/application
+
+A JupyterLab package that provides the top-level application object,
+with which JupyterLab plugins may be registered.

packages/docmanager-extension/README.md

@@ -1 +1,3 @@
 # @jupyterlab/docmanager-extension
+
+An extension for JupyterLab which provides an entry point and commands for the [@jupyterlab/docmanager](../docmanager) package.

packages/docmanager/README.md

@@ -1 +1,6 @@
 # @jupyterlab/docmanager
+
+A JupyterLab package which handles interactions with documents and the file system.
+It opens and closes files, creates new views onto files, and keeps track of open documents.
+
+The `DocumentManager` should be considered a singleton for the application.

packages/docregistry/README.md

@@ -1 +1,9 @@
 # @jupyterlab/docregistry
+
+A JupyterLab package which tracks the different types of documents that the application is able to interact with.
+This includes notebooks, text files, and base64 encoded documents.
+
+Extensions may register new document types with the document registry to allow them to be opened with JupyterLab.
+An example of this may be found in the [@jupyterlab/notebook](../notebook) package.
+
+The document registry is a singleton on the [application](../application).

packages/rendermime/README.md

@@ -6,3 +6,5 @@ HTML, images, and LaTeX.
 
 A simplified interface for adding new mime renderers to the application
 can be found in [@jupyterlab/rendermime-interfaces](../rendermime-interfaces).
+
+The rendermime is a singleton on the [application](../application).