Add alternate implementations for day 1, and create README of results.

2020/src/day1.rs

@@ -45,6 +45,8 @@ fn parse(input: &str) -> Vec<u32> {
 }
 
 /// Finds pairs of numbers in `nums` that sum to 2020.  If no pairs are found, the function panics.
+/// TODO(wathiede): make a version that sorts or uses a hash for finding the match to compare
+/// benchmarks.
 #[aoc(day1, part1, linear)]
 fn find_pair_2020_linear(nums: &[u32]) -> u32 {
     for (idx, first) in nums.iter().enumerate() {