update readme.

README.txt

@@ -1,10 +1,29 @@
-                               P5 IDEAS
-                Prototypes not yet part of the core API
+                                 ~~~
+
+                              p5 ideas
+                prototypes not yet part of the core API
 
                                  ~~~
 
-INCLUDED PROTOTYPES
+PROTOTYPES
+
+    - pfont : an implementation of the PFont API using the PIL
+      library.
+
+
+USAGE
+
+    All prototype-specific dependencies are listed in the
+    `prototype-name/requirements.txt` file. Test / example scripts
+    usually follow the naming convention `prototype-name-test.py`
+    `prototype-name-example.py` respectively.
+
+
+LICENSE
+
+    Licensed under the MIT License (see LICENSE.txt for details). Some
+    prototypes might use external code / resources. Refer to the
+    respective LICENSE and/or ATTRIBUTION files in the
+   `prototype-name/` folder for details.
 
 
-LICENSE 
-    See LICENSE.txt for details.