I have not tried it myself but this post http://crazyedy.com/tech/?p=535 claims that it is possible to add something like a readme after the DMG file has been created by making it writable.
Am 06.12.17 um 16:58 schrieb Mani Sarkar:
Hi, Has anyone worked out how to add a file like a README.txt into the DMG volume such that its not bundled into the app bundler (.app file) but placed outside it so that it is visible when the DMG volume is mounted for installation? It sounds like a simple action but the usage or docs don't point to it directly - from the options so far, it seems it might not be possible. How else can we present this to the end-user? Any thoughts on this? Thanks. Cheers, Mani