Create introduction for Python-TDD tutorial

Added an introduction to the Python-TDD tutorial, explaining TDD and its steps, along with the module structure.

Author: Carrington-dev

docs/introduction.rst

@@ -0,0 +1,68 @@
+Hello Python, Hi TDD!
+========================
+
+Introduction
++++++++++++++
+Hi and welcome to the Python-TDD tutorial where we are gonna learn **Python**
+by **TDD**. It's more advanced **Python** which means that you must
+have basic experience with this language.
+
+So here you will learn and practice how to write custom *decorator*,
+*iterator*, *context manager* etc. Hope that you will enjoy it.
+
+
+What is TDD?
+++++++++++++++
+`TDD (Test Driven Development) <https://en.wikipedia.org/wiki/Test-driven_development/>`_
+- is a software development process that relies on the repetition of a very short
+development cycle:  requirements are turned into very specific test cases, then the
+software is improved to pass the new tests, only.
+
+.. figure::  _static/images/tdd.png
+   :align:   center
+
+   TDD.
+
+How to start for TDD
+---------------------
+Here are the four steps required to do the TDD which one should remember by heart
+
+1. Write “enough” failing tests before writing the production code
+2. Write production code to pass those failing tests
+3. Re-factor the code, if required.
+4. GoTo Step — 1
+
+You are lucky
+---------------------
+Fortunately for you we have written tests for your tasks. So you will start your cycle
+from the *second* step. **[step-2, step-3, step-4]** *<next>* **[2, 3, 4]** *<next>*
+**[2, 3, 4]** ...
+
+
+Module structure
+++++++++++++++++++
+Every module have the next structure:
+::
+
+| module
+| ├── answers               (our implementation)
+| │   └── program.py
+| ├── tests                 (tests)
+| │   ├── test_program.py
+| ├── program.py            (your implementation)
+| ├── instruction.md        (what to do and how to run test)
+
+
+
++---------------+--------------+--------------------------------------------------------------------------+
+| Folder/File   | Example      | Description                                                              |
++===============+==============+==========================================================================+
+| ***.py**      | *program.py* | All ***.py** files in the root directory you have to implement.          |
+|               |              |                                                                          |
++---------------+--------------+--------------------------------------------------------------------------+
+| **tests/***   | *test_\*\.py*| Tests you have to run without failures.                                  |
+|               |              |                                                                          |
++---------------+--------------+--------------------------------------------------------------------------+
+| **answers/*** | *program.py* | Our implementation you can compare with your                             |
+|               |              |                                                                          |
++---------------+--------------+--------------------------------------------------------------------------+