Write initial version of the library.

README.md

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+# advent
+Collection of common code that may help in solving advent of code
+puzzles.
+
+## Installation
+
+1. Add the dependency to your `shard.yml`:
+
+   ```yaml
+   dependencies:
+     advent:
+       git: https://dev.danilafe.com/Advent-of-Code/advent
+   ```
+
+2. Run `shards install`
+
+## Usage
+
+```crystal
+require "advent"
+```