This shouldn't be done as this assumes the user wants mysql access as well. I do not have mysql and do not want mysql. thus the reason for a -mysql in my USE flag. because of this your README.authmysqlrc was not installed.
We don't write the readme files or the stock *rc files. The exact filename you ask for does not exist, and is not included in the source tarball. The closest name is README.authmysql.html which was not previously installed with the postgres use flag but is now added to all ebuilds, so the next time you sync and emerge the authlib, it will be in the /usr/share/doc/courier-authlib-*/html/ folder. Its the same file that can be read here though: http://www.courier-mta.org/authlib/?README.authmysql.html
