Task Description: Parrot uses an extensive test suite to verify it's behavior and also to prevent regressions as things inside libparrot change. For the test suite to be an effective tool at preventing problems, we need to have high code coverage. In order to get good code coverage, we rely on a variety of tools to calculate coverage statistics. This process, and the requirements for it, are undocumented.

Write documentation explaining code coverage in Parrot to current and future Parrot developers. This should include a list of prerequisites, the method to calculate coverage statistics, and a short description of their use and benefits.

Steps to Complete:

1. Create a fork of parrot/parrot on Github
2. Create a new documentation file "coverage.pod" in the directory docs/dev/
3. Open a pull request on Github