Update readme for default outputDirectory setting

[jasonkaedingCGI]

Describe the bug
The README.md file states that the default outputDirectory setting is the project root directory, but code demonstrates that it will default to the Jest coverageDirectory setting. It is a good thing that it does default to coverageDirectory, this allows me to configure the coverageDirectory in my CI build pipeline and everything works great and ends up in the right place!

To Reproduce
Steps to reproduce the behavior:

1. Set up the jest-sonar reporter. Do NOT configure the outputDirectory setting.
2. Configure the coverageDirectory to something far away from the project such as C:\test-results
3. Run jest tests
4. Look for the output file in coverageDirectory

Expected behavior
PLEASE DON'T CHANGE THE ACTUAL BEHAVIOR! It is useful as-is. Just update the docs so users are not confused.

Desktop (please complete the following information):

• OS: Windows
• NodeJS version 12.x

Additional context
Just a documentation update. I noticed how the code behaves by looking at https://github.com/sh33dafi/jest-sonar/blob/master/src/jest-sonar.js line 43.