docs: add crate section

Author: fspoettel

README.md

@@ -158,3 +158,10 @@ Go to the _Secrets_ tab in your repository settings and create the following sec
 
 > **Note**  
 > The session cookie might expire after a while (~1 month) which causes the automated workflow to fail. To fix this issue, refresh the `AOC_SESSION` secret.
+
+## Useful crates
+
+-   [itertools](https://crates.io/crates/itertools): Extends iterators with extra methods and adaptors. Frequently useful for aoc puzzles.
+-   [hashbrown](https://crates.io/crates/hashbrown): Faster hashing functions for `HashMap` and `HashSet`.
+
+Do you have aoc-specific crate recommendations? [Share them!](https://github.com/fspoettel/advent-of-code-rust/edit/master/README.md)