Add hint that using custom firmware files only works with a bootrom exploit

NEWS

@@ -8,7 +8,7 @@ Version 1.0.0
   - Updates the device by default or allows full restore erasing all data
   - Download latest available firmware for device on demand
   - Cache downloaded firmware files
-  - Restore using custom firmware files
+  - Restore using custom firmware files (requires bootrom exploit)
   - Skip NOR/Baseband upgrade
   - Fetch TSS records as ".shsh" files
   - Put devices in pwned DFU mode (limera1n devices only)

src/idevicerestore.c

@@ -120,7 +120,7 @@ static void usage(int argc, char* argv[], int err)
 	"  -v, --version         Print version information\n" \
 	"\n" \
 	"Advanced/experimental options:\n"
-	"  -c, --custom          Restore with a custom firmware\n" \
+	"  -c, --custom          Restore with a custom firmware (requires bootrom exploit)\n" \
 	"  -s, --cydia           Use Cydia's signature service instead of Apple's\n" \
 	"  -x, --exclude         Exclude nor/baseband upgrade\n" \
 	"  -t, --shsh            Fetch TSS record and save to .shsh file, then exit\n" \