Replaced Google Group by Stack Overflow and GIS SE links
Replaced Google Group by Stack Overflow and GIS SE links

in CONTRIBUTING.md following Leaflet/Leaflet#4055.

frogcat/Leaflet,hyperknot/Leaflet,Leaflet/Leaflet,tsantacruz/RotationalMarker,stefanocudini/Leaflet,iOffice/Leaflet,davojta/Leaflet,fab1an/Leaflet,frogcat/Leaflet,msiadak/Leaflet,msiadak/Leaflet,codeator/Leaflet,tsantacruz/RotationalMarker,iZucken/Leaflet,nathancahill/Leaflet,akrzyzanowskino/TeamSE,stefanocudini/Leaflet,w8r/Leaflet,nathancahill/Leaflet,jblarsen/Leaflet,Leaflet/Leaflet,jblarsen/Leaflet,jasonoverland/Leaflet,jblarsen/Leaflet,w8r/Leaflet,CapeSepias/Leaflet,codeator/Leaflet,davojta/Leaflet,Neorth/Leaflet,geoloep/Leaflet,scaddenp/Leaflet,Neorth/Leaflet,iOffice/Leaflet,Ubisense/Leaflet,brah/Leaflet,fab1an/Leaflet,davojta/Leaflet,rlugojr/Leaflet,Neorth/Leaflet,scaddenp/Leaflet,Trufi/Leaflet,jasonoverland/Leaflet,bbecquet/Leaflet,rlugojr/Leaflet,iZucken/Leaflet,Leaflet/Leaflet,codeator/Leaflet,w8r/Leaflet,jasonoverland/Leaflet,nathancahill/Leaflet,scaddenp/Leaflet,iOffice/Leaflet,Trufi/Leaflet,hyperknot/Leaflet,bbecquet/Leaflet,sisou/Leaflet,AP-whitehat/Leaflet,brah/Leaflet,stefanocudini/Leaflet,bbecquet/Leaflet,AP-whitehat/Leaflet,iZucken/Leaflet,sisou/Leaflet,brah/Leaflet,msiadak/Leaflet,AP-whitehat/Leaflet,geoloep/Leaflet,CapeSepias/Leaflet,rlugojr/Leaflet,Ubisense/Leaflet,frogcat/Leaflet,geoloep/Leaflet,Trufi/Leaflet,hyperknot/Leaflet,akrzyzanowskino/TeamSE,akrzyzanowskino/TeamSE,Ubisense/Leaflet,sisou/Leaflet,tsantacruz/RotationalMarker,CapeSepias/Leaflet,fab1an/Leaflet