Update Readme with doc link and Pypi installation

[FrancisBlais]

Update Readme

Update the README file with the following:

• Link to online documentation in the Documentation section
• Proper installation guide with Pypi in the documentation with mandatory tests when installing
• Change the command line to install the package using pip. The current line is pip install -r requirements.txt -e ./QDecomp. It should be pip install QDecomp when released on PyPi.