[doc] add a note regarding the use of plugins in mig-runner
[doc] add a note regarding the use of plugins in mig-runner

yvesago/mig,sunnygkp10/mig,rezen/mig,therealkbhat/mig,gdestuynder/mig,rezen/mig,yvesago/mig,pap/mig,vladimirdiaconescu/mig,vladimirdiaconescu/mig,mvanotti/mig,mvanotti/mig,gdestuynder/mig,mozilla/mig,pap/mig,therealkbhat/mig,vladimirdiaconescu/mig,pap/mig,mozilla/mig,mvanotti/mig,sunnygkp10/mig,rezen/mig,therealkbhat/mig,ameihm0912/mig,ameihm0912/mig,sunnygkp10/mig,ameihm0912/mig,jboyer2012/mig,therealkbhat/mig,jboyer2012/mig,yvesago/mig,sunnygkp10/mig,rezen/mig,jboyer2012/mig,jboyer2012/mig,yvesago/mig,pap/mig,mozilla/mig,vladimirdiaconescu/mig,mozilla/mig,gdestuynder/mig,mozilla/mig,mvanotti/mig,gdestuynder/mig