Add some documentation

src/jupyter_contrib_nbextensions/nbextensions/freeze/readme.md

@@ -1,13 +1,24 @@
 # Freeze
 
-This extension allows to make cells read-only or frozen.
+This extension allows to make cells read-only or frozen. It provides three buttons:
+* unlock
+* read-only
+* frozen
 
-If a code cell is read-only, it can be executed, but its input cannot be changed.
-Frozen code cells cannot be either altered or executed.
 
-If a markdown cell is read-only, its input can be viewed by double-clicking on it, but cannot be changed.
-Frozen markdown cells' input cannot be viewed by double-clicking.
+For **code-cells**:<br>
+_read-only_:  it can be executed, but its input cannot be changed.<br>
+_frozen_: It cannot be either altered or executed.
 
-To change cells' state, select them and press corresponding button.
+For **markdown-cells**:<br>
+_read-only_: It's input can be viewed by double-clicking on it, but cannot be changed.<br>
+_frozen_:  Input cannot be viewed by double-clicking.
 
-Cell's state is stored in its metadata and is applied to the cell if the extension is loaded.
+To change the state of a selected cell, press the corresponding button.
+
+The individual cell's state is stored in its metadata and is applied to the cell if the extension is loaded.
+
+##Internals
+
+The _read-only_ state is stored in the `cell.metadata.run_control.read_only` attribute.
+The _frozen_ state is stored in the `cell.metadata.run_control.frozen`attribute.