My Google search at first only lead me to a massive discussion thread on github (https://github.com/JuliaLang/julia/issues/69). While certainly insightful and interesting, I think it is convenient for many to have this question answered immediately on stackoverflow.
Multiline comments can be written by putting #=
before the comment and =#
after the comment. Like so
#=
enter comment here
=#
Note that these are nestable as well.