Add instructions about Transifex translations contributions
Add instructions about Transifex translations contributions

ahmadassaf/libreboard,ncarlier/libreboard,xsuperbug/libreboard,ahmadassaf/libreboard,xsuperbug/libreboard,guiquanz/libreboard,ArthurHlt/libreboard-cf,ArthurHlt/libreboard-cf,ncarlier/libreboard,ArthurHlt/libreboard-cf,ahmadassaf/libreboard,guiquanz/libreboard