Add README.

README.md

+# Mednafen PSX libretro
+
+This is port of Mednafen PSX core to the libretro API.
+It currently runs on Linux and possibly OSX.
+
+## Running
+
+To run this core, a proper configuration for Mednafen must be set up in
+$HOME/.mednafen as Mednafen itself expects.
+Here, at least the BIOS must be set up as it is not distributed.
+You should attempt to at least get an ISO image running in the real mednafen before attempting
+to use this port.
+
+
+## Loading ISOs
+
+Mednafen differs from other PS1 games in that it reads a .cue sheet that points to an .iso.
+If you have e.g. <tt>foo.iso</tt>, you should create a foo.cue, and fill this in:
+
+    FILE "foo.iso" BINARY
+       TRACK 01 MODE1/2352
+          INDEX 01 00:00:00
+
+After that, you can load the foo.cue file as a ROM.
+