blink: Update readme

blink/README.md

@@ -6,10 +6,35 @@ $ cargo build --release
 $ avrdude -F -patmega32u4 -cavr109 -P/dev/tty.usbmodem144101 -b57600 -D -Uflash:w:target/atmega32u4/release/blink.elf:e
 ```
 
+## `.cargo/config.toml`
+
+<https://doc.rust-lang.org/cargo/reference/config.html>
+
+### `env.AVR_CPU_FREQUENCY_HZ`
+
+README.md in [crates:avr-config]
+
+> This crate aims to provide the boilerplate for getting the CPU frequency as an integer at runtime, as well as establishes a convention that $AVR_CPU_FREQUENCY_HZ is used to pass the target frequency to all AVR crates, if they opt-in to it.
+
+### `unstable.build-std = ["core"]`
+
+<https://book.avr-rust.com/003.2-note-about-rust-build-std-flag.html>
+
+> AVR-Rust is not distributed with a pre-built libcore crate. Instead, it is compiled on-demand when a crate uses it via the Rust -Z build-std flag.
+>
+> There are many hundreds of variants of AVR microcontroller, and it is not feasible to distribute runtime libraries for all of them within a regular Rust distribution.
+>
+> Due to this, any time a crate is built for AVR, -Z build-std=core should be passed to cargo.
+
+<https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#build-std>
+
 ## TODOs
 
-- ☑ [crates:rduino] の trait を利用する
+- ☑ [crates:ruduino] の trait を利用する
+- ☑ `.cargo/config.toml` について調べる
 - data-layout が怪しい
 - target の json のファイル名が怪しい
-- `.cargo/config.toml` について調べる
 - embedded-hal との差を調べる
+
+[crates:ruduino]: https://crates.io/crates/ruduino
+[crates:avr-config]: https://crates.io/crates/avr-config