feat(intro): Add intro section.

exercises/intro/README.md

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+# Intro
+
+Rust uses the `print!` and `println!` macros to print text to the console.
+
+## Further information
+
+- [Hello World](https://doc.rust-lang.org/rust-by-example/hello.html)
+- [Formatted print](https://doc.rust-lang.org/rust-by-example/hello/print.html)

exercises/intro/intro1.rs

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+// intro1.rs
+// About this `I AM NOT DONE` thing:
+// We sometimes encourage you to keep trying things on a given exercise, even
+// after you already figured it out. If you got everything working and feel
+// ready for the next exercise, remove the `I AM NOT DONE` comment below.
+// Execute the command `rustlings hint intro1` for a hint.
+
+// I AM NOT DONE
+
+fn main() {
+    println!("Hello and");
+    println!(r#"       welcome to...                      "#);
+    println!(r#"                 _   _ _                  "#);
+    println!(r#"  _ __ _   _ ___| |_| (_)_ __   __ _ ___  "#);
+    println!(r#" | '__| | | / __| __| | | '_ \ / _` / __| "#);
+    println!(r#" | |  | |_| \__ \ |_| | | | | | (_| \__ \ "#);
+    println!(r#" |_|   \__,_|___/\__|_|_|_| |_|\__, |___/ "#);
+    println!(r#"                               |___/      "#);
+}

exercises/intro/intro2.rs

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+// intro2.rs
+// Make the code print a greeting to the world.
+// Execute `rustlings hint intro2` for a hint.
+
+// I AM NOT DONE
+
+fn main() {
+    println!("Hello {}!");
+}