Add pipx installation method

A great alternative to running coala in Docker or in a virtualenv environment is [pipx](https://pipxproject.github.io/pipx/). It lets you install python packages in an isolated environment, so you can use it without having to activate a virtual environment or installing directly in python.

pipx is great for CLI tools, so I use it for things like awscli, black, cookiecutter, pipenv, etc.

Coala doesn't install with the basic pipx install command, as you can see here: https://github.com/pipxproject/pipx/issues/14. I've confirmed that `pipx install --spec coala-bears coala` mentioned in the issue works for me, too.

I'm also happy to submit a pull request, if you want to add the instructions.