add readme

README.md

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+# Advent of Code 2020
+
+Solutions to [all 25 puzzles](https://adventofcode.com/2020) (with the inputs I was given).
+
+Run program for a specific day (e.g. day 25):
+
+    cargo run --release --bin day25
+
+Run and time all solutions:
+
+    cargo build --release
+    /usr/bin/time sh -c 'for d in `seq 1 25`; do echo "----- DAY $d -----"; ./target/release/day$d; echo "----- END DAY $d -----"; done'