add README.md

Author: remi-dupre

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+Advent of Code helper
+=====================
+
+This crate provides a macro to generate a handful main for your [advent of
+code][1] participation. The intention is to provide something similar to
+[cargo-aoc][2] through a much simple code base.
+
+![screenshoot](.screenshoot.png)
+
+Currently this will only provide a few benefits:
+
+  - a somewhat pretty orchestration of your solutions
+  - automatic download and loading of your input files
+
+
+Usage
+-----
+
+The generator and solutions for a given day must be implemented in a module
+called `dayXX`. Then you can simply invoke the `aoc::main!` macro in your
+`main.rs`:
+
+```rust
+mod day1 {
+    pub fn generator(input: &str) -> Vec<u64> {
+        input
+            .lines()
+            .map(|line| line.parse().unwrap())
+            .collect()
+    }
+
+    pub fn part_1(input: &[u64]) -> u64 {
+        input.iter().map(|&mass| mass / 3 - 2).sum()
+    }
+
+    pub fn part_2(input: &[u64]) -> u64 {
+        fn total_needed_mass(obj: u64) -> u64 {
+            if obj < 9 {
+                0
+            } else {
+                let obj_mass = obj / 3 - 2;
+                obj_mass + total_needed_mass(obj_mass)
+            }
+        }
+
+        input.iter().copied().map(total_needed_mass).sum()
+    }
+}
+
+mod day2;
+
+aoc::main! {
+    year 2019;
+    day1 : generator => part_1, part_2;
+    day2 : generator => part_1, part_2, part_2_optimized;
+}
+```
+
+Then you can simply run `cargo run`!
+
+Note that your solutions must take a borrowed version of the generator's
+output. Also, the generator can output a structure that contains borrowed data
+from the original input file.
+
+
+Fetching your AOC token
+-----------------------
+
+When the command line asks you for your session token, you can apply the
+following procedure (quoted from [cargo-aoc][2]):
+
+  - Firefox: "Storage" tab, Cookies, and copy the "Value" field of the session
+    cookie.
+  - Google Chrome / Chromium: "Application" tab, Cookies, and copy the "Value"
+    field of the session cookie.
+
+
+[1]: https://adventofcode.com
+[2]: https://github.com/gobanos/cargo-aoc