Simplify steps in usage section of README.md as now Cargo configuration works

Author: jhg

README.md

@@ -12,6 +12,7 @@ Designed for the ATmega328p.
 
   * A recent version of the nightly Rust compiler. Anything including or
     greater than `rustc 1.63.0-nightly (fee3a459d 2022-06-05)` can be used.
+  * A recent version of Cargo. At least 1.52.0 or greater.
   * The rust-src rustup component - `$ rustup component add rust-src`
   * AVR-GCC on the system for linking
   * AVR-Libc on the system for support libraries
@@ -26,12 +27,7 @@ Now to build, run:
 export AVR_CPU_FREQUENCY_HZ=16000000
 
 # Compile the crate to an ELF executable.
-cargo build -Z build-std=core --target avr-atmega328p.json --release
-
-# Note: there is work in progress to make that easier
-#       peeking in .cargo/config.tom  will show that
-#            cargo build --release
-#       is the short version
+cargo build --release
 ```
 There should now be an ELF file at `target/avr-atmega328p/release/blink.elf`. It
 can be flashed directly to an AVR microcontroller or ran inside a simulator.