Add illustration of using the / and * in functions

Author: gjbex

source-code/README.md

@@ -6,6 +6,7 @@ was used to develop it.
 
 ## What is it?
 
+1. `api-design`: illustrates how to design APIs in Python.
 1. `context-manager`: illustrates how to write your own context managers.
 1. `coroutines`: illustrates how to write coroutines in Python.
 1. `decorators`: illustrates how to write decorators in Python.

source-code/api-design/README.md

@@ -0,0 +1,14 @@
+# API design
+
+Designing an API is an important part of the software development process. It
+is the contract that defines how software components are used and interact. A
+well-designed API helps developers as well as users to understand the
+functionality of the software and how to use it.  It also contributes to the
+maintainability and scalability of the software.
+
+
+## What is it?
+
+1. `positional_vs_keyword_arguments.ipynb`: Jupyter notebook that illustrates
+   the difference between positional and keyword arguments in Python, and how
+   to enforce the use of either.