See the detailed results of the Monmouth University poll here (pdf).
Notably, Johnson polled 11% in spite of name recognition of just 25%, meaning that among voters who know who he is, Johnson polled 44%. This means it is very likely that his poll numbers will increase as media coverage of the campaign begins to snowball and he is included in additional polls.
Also noteworthy, is that Johnson’s inclusion hurts Hillary Clinton (-6 points) slightly more than Donald Trump (-4 points), despite Johnson performing better in states won by Romney in 2012 (15%) than states won by Obama (10%), and in spite of the fact that Johnson was polling at 16% among independents, 13% among Republicans, but just 4% of Democrats.
This strongly suggests that Johnson’s strongest base of support so far, outside of independents and Libertarians, is among the sizable number of Republicans who would otherwise vote for Hillary Clinton over Donald Trump.