it is recommended for the newcomers not to play MGS5 straight yet, till the they got the plots of the pervious games. They wouldnt got anything from the storyline and they will enjoy nothing.
To catch up with MGS5 soon, it is recommended not to play the whole pervious games, it would take so long.
at least, watch the videos over youtube either cutscens or walkthru in order to get the meaning of this game.
Then try out & enjoy MGS5.