document firered’s mysterious build failure

README.md

@@ -22,6 +22,15 @@ with the environment variable `NIXPKGS_ALLOW_UNFREE=1` set.
 
 You can find a list of all packages provided by this overlay in `default.nix`.
 
+## FAQ
+
+### FireRed fails to build
+
+For an unknown reason, FireRed fails to build sometimes without an error at
+`arm-none-eabi-objcopy -O binary --gap-fill 0xFF --pad-to 0x9000000
+pokefirered_rev1.elf pokefirered_rev1.gba`. Restarting the build a few times
+should produce a succeeding build.
+
 ## License
 
 [The MIT License](./LICENSE)