Explain where the final ELF final goes in the README

Author: dylanmckay

README.md

@@ -18,3 +18,5 @@ Then run:
 cargo build --target avr-atmega328p.json -Z build-std=core --release
 ```
 
+The final ELF executable file will then be available at `target/avr-atmega328p/release/template-bin.elf`.
+