pd777: zip file support

docs/library/pd777.md

@@ -29,10 +29,13 @@ Minimal.
 
 ## How to start the PD777 core:
 
-ROM file handling is currently awkward, but gives some options that work.
+ROM file handling is slightly awkward because of the two-file ROM format.
 
-1. Supply a .bin777 file to load; the core looks for a similarly-named .ptn777 file in the same directory.
-2. If that fails, run the content compiled from core/rom.cpp (as in standalone emulator).
+1. Zip-files (bundling one `.bin777` and one `.ptn777`) are supported.
+1. Supply a `.bin777` file to load; the core looks for a similarly-named `.ptn777` file in the same directory.
+
+There is not yet a suitable single-file format for the Cassette Vision.
+If ROM loading fails, the core will run a built-in balloon demo, as in standalone emulator.
 
 ## BIOS
 
@@ -42,6 +45,7 @@ None.
 
 Content that can be loaded by the PD777 core have the following file extensions:
 
+- .zip
 - .bin777
 
 RetroArch database(s) that are associated with the PD777 core:
@@ -97,7 +101,7 @@ Standard.
 
 ## Core options
 
-None yet.
+Input > Announce Course Switch: Show the new setting of the Course Select Switch when changing it with up/down buttons.
 
 ## User 1 device types