CONTRIBUTING.md: ask for unmodified ZNC
CONTRIBUTING.md: ask for unmodified ZNC

Specify unmodified ZNC in case the issue is caused by modified ZNC (such
as
the SSL certificate validation check removal or firre webadmin theme
(see znc/znc#384 where after fixing patches have reintroduced it)).

<s>Thanks to @kerio92 & @Zarthus for wording suggestions.</s>
DarthGandalf reworded it.

[CI SKIP]

Mkaysi/znc,anthonyryan1/znc,Mikaela/znc,BtbN/znc,aarondunlap/znc,DarthGandalf/znc,Mikaela/znc,Mikaela/znc,Phansa/znc,Phansa/znc,Zarthus/znc,jreese/znc,KielBNC/znc,BtbN/znc,evilnet/znc,Mkaysi/znc,dgw/znc,evilnet/znc,jpnurmi/znc,aarondunlap/znc,dgw/znc,dgw/znc,psychon/znc,evilnet/znc,anthonyryan1/znc,GLolol/znc,aarondunlap/znc,kashike/znc,anthonyryan1/znc,Mkaysi/znc,GLolol/znc,kashike/znc,Phansa/znc,wolfy1339/znc,kashike/znc,psychon/znc,anthonyryan1/znc,Adam-/znc,markusj/znc,wolfy1339/znc,jpnurmi/znc,Adam-/znc,Zarthus/znc,DarthGandalf/znc,Zarthus/znc,badloop/znc,GLolol/znc,Mkaysi/znc,GLolol/znc,kashike/znc,DarthGandalf/znc,elyscape/znc,dgw/znc,elyscape/znc,Mkaysi/znc,jpnurmi/znc,znc/znc,Phansa/znc,Adam-/znc,DarthGandalf/znc,elyscape/znc,elyscape/znc,aarondunlap/znc,BtbN/znc,aarondunlap/znc,badloop/znc,KielBNC/znc,jreese/znc,BtbN/znc,DarthGandalf/znc,GLolol/znc,anthonyryan1/znc,Mikaela/znc,Zarthus/znc,psychon/znc,elyscape/znc,BtbN/znc,jpnurmi/znc,Mikaela/znc,Adam-/znc,dgw/znc,anthonyryan1/znc,markusj/znc,Adam-/znc,DarthGandalf/znc,KielBNC/znc,Mikaela/znc,evilnet/znc,wolfy1339/znc,badloop/znc,jreese/znc,GLolol/znc,Mkaysi/znc,Adam-/znc,Zarthus/znc,kashike/znc,evilnet/znc,znc/znc,wolfy1339/znc,jpnurmi/znc,Mkaysi/znc,znc/znc,BtbN/znc,badloop/znc,jreese/znc,markusj/znc,znc/znc,psychon/znc,wolfy1339/znc,psychon/znc,KielBNC/znc,dgw/znc,znc/znc,aarondunlap/znc,badloop/znc,jpnurmi/znc,markusj/znc,jreese/znc,markusj/znc,KielBNC/znc,badloop/znc,wolfy1339/znc,jpnurmi/znc,Phansa/znc,Zarthus/znc,badloop/znc,markusj/znc,anthonyryan1/znc,psychon/znc,Mikaela/znc,jreese/znc,aarondunlap/znc,KielBNC/znc,elyscape/znc,znc/znc,Phansa/znc,kashike/znc,Adam-/znc,evilnet/znc