docs(usage): describe app0.zar

docs/user/USAGE.md

@@ -107,6 +107,8 @@ Use `sce_modules/` or `prx/` instead of `sce_module/` if that is the input direc
 
 Some applications also need their original `app0/sce_sys/param.json` at runtime. AnyPS5 reads this file for the title ID and display name; it must contain a non-empty `titleId` string and a non-empty `localizedParameters` object with a language entry that has a non-empty `titleName`. It selects the entry named by `defaultLanguage` (inside `localizedParameters`, or at the top level); if that entry is absent, it tries `en-US`, then the first object-valued language entry. Preserve the values from the title's own metadata and place the file under the same `app0/` directory as the other app resources. At startup, its presence in the working directory creates `data/`; a non-zero `downloadDataSize` also creates `download0/`. Without the file, neither directory is created. If it is missing or invalid, metadata calls such as title/window setup can fail with a `param.json` error.
 
+Resources can also be packed into `app0.zar` beside `app0/`, a [ZArchive](https://github.com/Exzap/ZArchive) whose root is the `app0/` directory. A file present in `app0/` is read from there and any other file from the archive; the executable, `sce_module/` and `sce_sys/` stay in `app0/`. The archive is read only through `sceKernelOpen`/`open`, the read, seek and stat calls on its descriptors and `sceKernelStat`; other calls on an archived path, listing a directory that the archive also contains and opening a file in it for writing fail as described in [technical debt](../dev/debt/libkernel.md). An archive that cannot be opened stops the title at its first file access.
+
 Use the generated files printed as `Guest module:` for bundled title modules. `libs/` is for AnyPS5 system libraries, not the original PS5 `.prx` files. Placing an original PS5 module in `libs/` on Windows makes Windows try to load it as a DLL and can fail with error 193 (not a valid Win32 application).
 
 On Windows, a self-built `libs/` also needs `libgcc_s_seh-1.dll`, `libstdc++-6.dll` and `libwinpthread-1.dll` from the `mingw64/bin` directory of the toolchain that built the libraries; the release archives already contain them. The libraries are loaded without searching `PATH`, so a copy elsewhere on the system is not used, and a library that needs one of them fails with error 126 (the specified module could not be found) although its `.prx` file is present.