Hydra already saw Doctor Strange as a threat as evidenced by a line of dialogue in Captain America: The Winter Soldier when he was targeted for assassination. Does that mean he's just a potentially problematic neurosurgeon or was the character already on his way to becoming the Sorcerer Supreme in that part of the continuity?
Traditionally, the stories of Marvel movies take place in chronological order of their release, but that may not always be the case. Whether or not Strange is already a super hero during the events of Civil War it'd make sense to introduce him during a Captain America: Civil War post-credits button sequence since Doctor Strange is the other 2016 movie release.
Doctor Strange begins production in London this fall where Benedict Cumberbatch will suit up in a classic Marvel Comics inspired costume. That leaves plenty of time for a button sequence to be shot and slotted at the end of Captain America: Civil War before it opens in theaters next May.