Update READMEs for 2021.

2021/README.md

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+# Advent of Code 2021
+
+This folder contains the solution runner for Advent of Code 2021. All days will be solved in Rust,
+with the goal of having a total time across all puzzles of one second or less.
+
+```
+aoc-2021 
+
+Advent of Code 2021 runner
+
+USAGE:
+    aoc-2021 [OPTIONS] <DAY>
+
+ARGS:
+    <DAY>    Which day to run
+
+OPTIONS:
+    -2, --part2            Run part 2 instead of part 1
+    -h, --help             Print help information
+    -i, --input <INPUT>    Read input from the given file instead of stdin
+    -t, --time             Print time taken
+```