more detailed instructions for `pulp`/`cbc` install

[jGaboardi]

Following up from #347, we should add more detailed instructions for pulp/cbc installation in:

• README.md
• docs/installation.rst

cc @qszhao

[gegen07]

From the discussion on PR. Is it acceptable to loop over the solver available list with a priority list like (get GUROBI if it is available, then COIN, then...). I think PULP has a default installation, but unfortunately, we cannot set the default solver to be installed.

This solution has some warnings and errors to be raised. If the desired solver is not installed, we can raise a warning. If no solver is installed, we raise an error.

This sounds good? @jGaboardi @ljwolf

Happy to open a PR, if it is ok.

[qszhao]

I think this is a good idea since pulp/cbc can be the default installation on different platforms.

[jGaboardi]

@gegen07 Yeah, let's try it. But I think we should stick with open source options as a top defaults.