Document common issues

[sebffischer]

• Need to install package for encapsulation test to work
• check_man also required package installation
• run the setup.R file for the tests to run faster
• include mlr3extralearners in the packages field

[bblodfon]

• add minimum working version for a package in DESCRIPTION.md which is the latest GitHub/CRAN upon contributing/creating the learner and update the version when things break and maintainer fixes them

[bblodfon]

@sebffischer I guess the best place to put these is here? => https://mlr3extralearners.mlr-org.com/articles/extending.html#contributing-to-mlr3extralearners