docs: asset naming convention

[setchy]

closes #92

[erikian]

suggestion: same here with the default arch, I feel like we could move the contents of Special cases into Windows Assets and make it more consistent with macOS Assets.

Also a note: currently, the {{appName}}-win32-{{arch}}.*.exe pattern seems to be correctly recognized for all architectures, so in the code below I updated the extension requirements and included more examples.

### Windows Assets
- Asset must be a `.zip` or `.exe` file.
- Asset name must include the `-win32` platform identifier.
- Asset name may specify the architecture (if not specified, will default to `-x64`):
  - `-ia32` for 32-bit Windows.
  - `-x64` for 64-bit Windows.
  - `-arm64` for ARM-based Windows.
- `.exe` files without specific architecture tags or the `-win32` platform identifier are assumed to be `-x64` by default.

**Example asset names:**
- `app-win32-ia32.zip`
- `app-win32-x64.zip`
- `app-win32-arm64.zip`
- `app-win32-arm64.exe`
- `app-win32-arm64-v1.2.3.exe`
- `app-win32.exe` (no architecture specified - treated as `-x64`)
- `app-installer.exe` (generic `.exe` file with no architecture or platform identifier specified - treated as `-x64`)

We might want to update the error message for PLATFORM.WIN32 to something like No updates found (needs asset containing .*-win32-(x64|ia32|arm64).*.(zip|exe) or *.exe in public repository) in a follow-up PR.

Not sure about this one so I'll ask others in @electron/wg-ecosystem, but we might also want to lift the requirement for the -win32 platform specifier for .exe files - I assume this requirement is in place as a way to avoid confusion between .zip files for Windows and macOS, but that's not an issue with .exe files.

[setchy]

incorporated the suggested changes to the readme.

[erikian]

Thanks for the contribution!