Add readme to javascript package (#106)

Author: fcollonval

javascript/README.md

@@ -0,0 +1,8 @@
+# @jupyter/ydoc
+
+`@jupyter/ydoc` provides [Yjs](https://github.com/yjs/yjs)-based data structures for various
+documents used in the Jupyter ecosystem. Built-in documents include:
+- `YFile`: a generic text document.
+- `YNotebook`: a Jupyter notebook document.
+
+The API documentation is available [there](https://jupyter-ydoc.readthedocs.io/en/latest/api/index.html).