Contribution guide: added "what is pull request" section
Contribution guide: added "what is pull request" section

eli-b/pytest,ghostsquad/pytest,ghostsquad/pytest,skylarjhdownes/pytest,mhils/pytest,ojake/pytest,Akasurde/pytest,MichaelAquilina/pytest,mdboom/pytest,alfredodeza/pytest,nicoddemus/pytest,tomviner/pytest,JonathonSonesen/pytest,omarkohl/pytest,userzimmermann/pytest,The-Compiler/pytest,chiller/pytest,bubenkoff/pytest,ionelmc/pytest,tareqalayan/pytest,bukzor/pytest,gabrielcnr/pytest,vodik/pytest,MengJueM/pytest,oleg-alexandrov/pytest,rmfitzpatrick/pytest,ojake/pytest,mdboom/pytest,tgoodlet/pytest,vmalloc/dessert,oleg-alexandrov/pytest,pelme/pytest,wfxiang08/pytest,rouge8/pytest,JonathonSonesen/pytest,inirudebwoy/pytest,davidszotten/pytest,rouge8/pytest,jaraco/pytest,ionelmc/pytest,tomviner/pytest,ropez/pytest,omarkohl/pytest,hackebrot/pytest,untitaker/pytest,ropez/pytest,RonnyPfannschmidt/pytest,nicoddemus/repo-test,chillbear/pytest,ericdill/pytest,hunse/pytest,codewarrior0/pytest,Bjwebb/pytest,hpk42/pytest,mbirtwell/pytest,wfxiang08/pytest,mhils/pytest,pytest-dev/pytest,ddboline/pytest,takluyver/pytest,Bachmann1234/pytest,ericdill/pytest,vodik/pytest,The-Compiler/pytest,MengJueM/pytest,Carreau/pytest,txomon/pytest,doordash/pytest,malinoff/pytest,Haibo-Wang-ORG/pytest,bukzor/pytest,icemac/pytest,Carreau/pytest,codewarrior0/pytest,takluyver/pytest,etataurov/pytest,chillbear/pytest,Haibo-Wang-ORG/pytest,untitaker/pytest,bubenkoff/pytest,gabrielcnr/pytest,icemac/pytest,nicoddemus/repo-test,userzimmermann/pytest,jb098/pytest,pfctdayelise/pytest,flub/pytest,abusalimov/pytest,pelme/pytest,hunse/pytest,jb098/pytest,lukas-bednar/pytest,hpk42/pytest,inirudebwoy/pytest,doordash/pytest,Bachmann1234/pytest,markshao/pytest,nicoddemus/pytest,lukas-bednar/pytest,chiller/pytest,mbirtwell/pytest,abusalimov/pytest,Bjwebb/pytest