It's in the README file in the root of the pcsxrearmed source (and has been there since the first git commit on 2010-11-16).

"PCSX-Reloaded has a very capable Internal HLE BIOS that can run many games without problems. It is recommended that you use it. However, if you own a real PlayStation, you may be able to use your own BIOS image. PCSX-Reloaded will find it in ~/.pcsx/bios/ or /usr/share/psemu/bios/ if you place it there. This can improve compatibility, especially with certain games and with the use of memory cards."