clarified purpose of `pip install -e` command
clarified purpose of `pip install -e` command

ref #1937
pytest-dev/pytest,tomviner/pytest,The-Compiler/pytest,RonnyPfannschmidt/pytest,alfredodeza/pytest,skylarjhdownes/pytest,hackebrot/pytest,tgoodlet/pytest,jaraco/pytest,markshao/pytest,davidszotten/pytest,rmfitzpatrick/pytest,Akasurde/pytest,ddboline/pytest,MichaelAquilina/pytest,flub/pytest,nicoddemus/pytest,The-Compiler/pytest,txomon/pytest,nicoddemus/pytest,tomviner/pytest,pfctdayelise/pytest,tareqalayan/pytest