[PocketBook] Add Inkpad 3 (PB740) definition (#3830)
[PocketBook] Add Inkpad 3 (PB740) definition (#3830)

Fixes #3687

Thanks to @Maradar for testing, see https://github.com/koreader/koreader/issues/3687#issuecomment-378652074
mihailim/koreader,koreader/koreader,poire-z/koreader,koreader/koreader,Markismus/koreader,Frenzie/koreader,poire-z/koreader,pazos/koreader,houqp/koreader,NiLuJe/koreader,NiLuJe/koreader,lgeek/koreader,Hzj-jie/koreader,mwoz123/koreader,Frenzie/koreader