Simplify instructions using current Cargo features and reduce size of compiled result

Cargo.toml

@@ -8,4 +8,6 @@ edition = '2018'
 ruduino = {  git = "https://github.com/avr-rust/ruduino", branch = "master" }
 
 [profile.release]
+opt-level = 'z'
 lto = true
+strip = true

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
@@ -22,18 +23,11 @@ Designed for the ATmega328p.
 Now to build, run:
 
 ```bash
-rustup override set nightly
-
 # Ensure time delays are consistent with a 16MHz microcontroller.
 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.

rust-toolchain.toml

@@ -0,0 +1,2 @@
+[toolchain]
+channel = "nightly"