Add an example on how to update config.warn calls
Add an example on how to update config.warn calls

As commented in https://github.com/pytest-dev/pytest-cov/pull/230#pullrequestreview-157958838

nicoddemus/pytest,pytest-dev/pytest,tomviner/pytest,davidszotten/pytest,RonnyPfannschmidt/pytest,nicoddemus/pytest,ddboline/pytest,Akasurde/pytest,hackebrot/pytest,The-Compiler/pytest,The-Compiler/pytest,pfctdayelise/pytest,txomon/pytest,alfredodeza/pytest,markshao/pytest,tomviner/pytest