add readme

README.md

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+# libaoc
+A library for writing very rusty AdventOfCode solutions with miette error handling.
+
+## Example
+```rust
+fn main() -> libaoc::miette::Result<()> {
+    libaoc::run_days!(day1)
+}
+
+// This could also be a new file with #![...] at the start using nightly rust!
+#[libaoc::day(1, "../input/day1.txt")]
+mod day1 {
+    use libaoc::miette::Result;
+
+    fn part1() -> Result<()> {
+        println!("Running part 1. My input is '{}'", INPUT);
+
+        Ok(())
+    }
+
+    // This is optional.
+    fn part2() -> Result<()> {
+        println!("Running part 2. My input is '{}'", INPUT);
+
+        Ok(())
+    }
+}
+```
+
+This gives the output
+
+```
+Running day 1...
+--- Part 1 ---
+Running part 1. My input is 'day 1 input file here!'
+
+--- Part 2 ---
+Running part 2. My input is 'day 1 input file here!'
+```