Add main.rs to run all solutions and update README.md.

README.md

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+```
+Advent of code 2020
+Day 1 - Part 1: 1016964
+	generator: 19.462µs,
+	runner: 638ns
+
+Day 1 - Part 2: 182588480
+	generator: 13.395µs,
+	runner: 397ns
+
+Day 2 - Part 1: 586
+	generator: 133ns,
+	runner: 143.612µs
+
+Day 2 - Part 2: 352
+	generator: 182ns,
+	runner: 128.948µs
+
+Day 3 - Part 1: 265
+	generator: 70.461µs,
+	runner: 74.25µs
+
+Day 3 - Part 2: 3154761400
+	generator: 59.863µs,
+	runner: 398.929µs
+
+Day 4 - Part 2: 186
+	generator: 133ns,
+	runner: 156.514µs
+
+Day 5 - Part 1: SeatId(933)
+	generator: 194ns,
+	runner: 62.802µs
+
+Day 5 - Part 2: SeatId(711)
+	generator: 159ns,
+	runner: 70.564µs
+
+Day 6 - Part 1: 6633
+	generator: 158ns,
+	runner: 162.734µs
+
+Day 6 - Part 2: 3202
+	generator: 116ns,
+	runner: 138.78µs
+
+```

src/main.rs

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+use aoc_runner_derive::aoc_main;
+
+aoc_main! { lib = aoc }