It seems like LICENSE.md would be a good place to include information about the logo’s license / trademark / usage guidelines.
There also seems to be a few instances of the image in various repos:
- JuliaLang/julia: doc/src/assets/logo.png
- JuliaLang/JuliaDoc: juliadoc/theme/julia/static/julia-logo.svg
- JuliaLang/www.julialang.org:
Not sure if these are being kept in sync, and whether the png ones even should be there, considering JuliaLang/julia#4640.
I believe @StefanKarpinski is the right person to comment on this.