added readme

README.md

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+[![License](https://img.shields.io/badge/license-%20BSD--3-blue.svg)](../master/LICENSE)
+
+
+## Git bisect exercise
+
+### Motivation
+
+The motivation for this exercise is to be able to do archaeology with Git on a
+source code where the bug is difficult to see visually. **Finding the offending
+commit is often more than half the debugging**.
+
+
+### Background
+
+The script `get_pi.py` approximates pi using terms of the Nilakantha series. It
+should produce 3.14 but it does not. The script broke at some point and
+produces 3.57 using the last commit:
+
+```
+$ python get_pi.py
+
+3.57
+```
+
+At some point within the 500 first commits, an error was introduced. The only
+thing we know is that the first commit worked correctly.
+
+
+### Your task
+
+Clone or fork this repository and use `git bisect` to find the commit which
+broke the computation.