Fix documentation in PGauge SOURCE_CODE file

[timpokorny]

Summary

The documentation included in the SOURCE_CODE file provided with PGauge is not accurate now that we've moved it to GitHub. Update the file with better documentation providing a bit of an introduction the source code structure for the project.

Acceptance Criteria

• SOURCE_CODE document shipped with PGauge will be updated to provide an introduction to how the source code is structured and the way the framework hangs together
• SOURCE_CODE document will be converted to Markdown