I have a project which uses unmodified libraries with MIT-like licenses. I don't include source code of these libraries in my project tree, just include some its configuration files which have full text of its license in the header comment.
I want to release all parts of my project, not directly derived from this libraries, to public domain (e.g. with Unlicense).
Can I just use note like "unless otherwise noted this project is released under Unlicense" in LICENSE or README file.
You can write whatever you want, as long as you respect the licences of any external pieces you include/use.
On the other hand, you want any user of your code to be able to find out easily, and without any ambiguities, what is going on. It isn't that much of a hassle to add a README file (or a section thereof) spelling out what license(s) apply to which parts of the code, perhaps giving relevant URLs or including relevant licenses verbatim in separate files (perhaps in a LICENSESdirectory?). If you write it out or not, the work to clarify the status of each piece (just to make sure you don't commit any transgressions by using foreign code) has to be done anyway.
