Added .mkdn as a Markdown extension
Added .mkdn as a Markdown extension

I have already submitted a corresponding change to Linguist (see #1215)
HsuTing/markup,whatvn/markup,ya7lelkom/markup,sancolo/markup,SergeyPodgornyy/markup,supriyantomaftuh/markup,rgarro/markup,martynbm/markup1,jacobxk/markup,jacobxk/markup,furkanayhan/markup,betatim/markup,hernanramirez/markup,titiushko/markup,SergeyPodgornyy/markup,patidarr/markup,kblizeck/markup,wadya/markup,jaqe18/markup,RunYouCleverBoy/markup,jack-michaud/markup,amitai-hoze/markup,silviolleite/markup,pwz3n0/markup,JussMee15/markup,sonico999/markup,dherre3/markup,jaqe18/markup,github/markup,patidarr/markup,HsuTing/markup,HelgeKrueger/markup,amitai-hoze/markup,vheun/markup,martynbm/markup1,dherre3/markup,pchaigno/markup,silviolleite/markup,DuyBach/markup,SergeyPodgornyy/markup,achristodoulou/markup,jacobxk/markup,moreati/markup,gpabdo/markup,asiniy/markup,vheun/markup,sebadiaz/markup,sebadiaz/markup,essamjoubori/markup,RunYouCleverBoy/markup,marengaz/markup,jeffersonphp/markup,dherre3/markup,asiniy/markup,pwz3n0/markup,jaqe18/markup,bwaidelich/markup,rgarro/markup,jeremiahyan/markup,GamerKnight/markup,titiushko/markup,jeremiahyan/markup,nilehmann/markup,briandoconnor/markup,marengaz/markup,mortegac/markup,mortegac/markup,sorrycc/markup,sonico999/markup,sorrycc/markup,titiushko/markup,kblizeck/markup,AshikNesin/markup,wadya/markup,thekindofme/markup,liquorburn/markup,JussMee15/markup,sweet-t-nd/markup,luthfi401/markup,pchaigno/markup,rgarro/markup,hernanramirez/markup,GamerKnight/markup,moreati/markup,sancolo/markup,ya7lelkom/markup,sorrycc/markup,whatvn/markup,Rathachai/markup,sweet-t-nd/markup,HelgeKrueger/markup,bwaidelich/markup,JussMee15/markup,DuyBach/markup,peeka15/markup,briandoconnor/markup,marengaz/markup,silviolleite/markup,JoshDellay/markup,jayhetee/markup,luthfi401/markup,gpabdo/markup,pchaigno/markup,furkanayhan/markup,pchaigno/markup,github/markup,supriyantomaftuh/markup,essamjoubori/markup,thekindofme/markup,silvestrelosada/markup,silvestrelosada/markup,HsuTing/markup,betatim/markup,jbanety/markup,jack-michaud/markup,RunYouCleverBoy/markup,HelgeKrueger/markup,webdevl/markup,achristodoulou/markup,essamjoubori/markup,hernanramirez/markup,pwz3n0/markup,thekindofme/markup,jayhetee/markup,Rathachai/markup,jack-michaud/markup,github/markup,sancolo/markup,jbanety/markup,JoshDellay/markup,moreati/markup,amitai-hoze/markup,briandoconnor/markup,Rathachai/markup,betatim/markup,AshikNesin/markup,github/markup,asiniy/markup,ya7lelkom/markup,luthfi401/markup,sonico999/markup,wadya/markup,achristodoulou/markup,peeka15/markup,whatvn/markup,jeremiahyan/markup,jbanety/markup,bwaidelich/markup,Filebb/markup,supriyantomaftuh/markup,vheun/markup,patidarr/markup,peeka15/markup,JoshDellay/markup,sweet-t-nd/markup,AshikNesin/markup,Filebb/markup,liquorburn/markup,furkanayhan/markup,sebadiaz/markup,kblizeck/markup,DuyBach/markup,GamerKnight/markup,jeffersonphp/markup,jeffersonphp/markup,Filebb/markup,mortegac/markup,gpabdo/markup,martynbm/markup1,jayhetee/markup,webdevl/markup,nilehmann/markup,liquorburn/markup,webdevl/markup,silvestrelosada/markup,nilehmann/markup