the whole "echo in post_install" custom should be avoided. think of any gui - their stdout is even invisible in most cases
it's up to the Arch devs how they solve this problem - in Frugalware we use to add README.Frugalware files for packages and such infos are described there. probably such READMEs are not preferred by them since by default they remove all the docs from the packages
README can be good if it will be displayed automatically after package install. This is a job of any libalpm front-end anyway. It should be treated like .PKGINFO or .FILELIST files - I mean these README files should be used for front-end work only, and not installed anywhere. There is another thing - some packages generates some progress info during install (like ">>> Genreating initial ramdisk... done"). - this should be addressed too, maybe with something like status_message() function, or something else. By addressing this issues now we can avoid different custom "echo in post_install" things for different packages in different front-ends. I don't know much about libalpm/pacman3 internals yet, but I think that these issues can be addressed.