Mention that pytest_addoption may be used from conftest files
Mention that pytest_addoption may be used from conftest files

fix #1427

skylarjhdownes/pytest,hackebrot/pytest,vmalloc/dessert,vodik/pytest,tomviner/pytest,davidszotten/pytest,MichaelAquilina/pytest,ddboline/pytest,ropez/pytest,hpk42/pytest,ropez/pytest,The-Compiler/pytest,markshao/pytest,vodik/pytest,omarkohl/pytest,tareqalayan/pytest,hpk42/pytest,MengJueM/pytest,nicoddemus/pytest,MengJueM/pytest,pytest-dev/pytest,RonnyPfannschmidt/pytest,tgoodlet/pytest,userzimmermann/pytest,tomviner/pytest,eli-b/pytest,etataurov/pytest,JonathonSonesen/pytest,pfctdayelise/pytest,JonathonSonesen/pytest,flub/pytest,Akasurde/pytest,userzimmermann/pytest,The-Compiler/pytest,nicoddemus/pytest,txomon/pytest,Bachmann1234/pytest,malinoff/pytest,rmfitzpatrick/pytest,Bachmann1234/pytest,jaraco/pytest,omarkohl/pytest,alfredodeza/pytest