Documentation for the memory_leak Case Study

It would be very helpful to have some documentation for the [memory_leak](https://github.com/flutter/devtools/tree/master/case_study/memory_leaks/images_1) Case Study. There is code there but nothing to indicate how to use the tool to find the issues in the code.

The companion case study called [memory_leaks/images_1](https://github.com/flutter/devtools/tree/master/case_study/memory_leaks/images_1) has good documentation.