Function attach 

pub fn attach(object: &mut Vec<u8>, name: &str, payload: &[u8]) -> bool

Puts payload into object as a section called name, which the linker leaves out of what it writes. Whether it did, which it does not for a file that is not little endian ELF or that has too many sections to add one to.

Everything goes on the end. The new section header table is the old one with one more header after it, so no section changes its index and nothing that points at one by number has to be told, and the names of the sections are copied there too with the new one after them, since the old table of names has no room. The old copies of both are left where they were, which is bytes nothing points at any more.