Add driver, license and Readme.

README.md

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+# C++ Solutions to Advent of Code
+
+This repository contains solutions to various years of the [Advent of Code](https://adventofcode.com)
+puzzles.  All solutions are written in C++.  The solutions aren't designed to be nice or
+pretty.  Instead, I am using it as an experiment in understanding how I approach solving
+problems.
+
+The solutions are authored by Matthew Gretton-Dann, and Copyright 2020-2021, Matthew Gretton-Dann.  Licensed under Apache 2.0 - see [license](./LICENSE).
+
+## Getting the sources
+
+To get the sources clone from git as follows:
+
+```sh
+git clone https://github.com/matt-gretton-dann/advent-of-code/
+```
+
+## Build requirements
+
+The following is required to build the puzzles:
+
+ * CMake
+ * C++20 compiler
+ * OpenSSL headers.
+
+## Running the Puzzles
+
+There is a driver script (`./driver.sh`) in the root of the checkout.  It will build and run specific tests.  Example
+use is as follows:
+
+```sh
+YEAR=2020 # Year of puzzle to run
+DAY=01 # Day of puzzle to run
+PUZZLE=01 # ID of puzzle (1 or 2)
+INPUT=input1.txt # Input to give the puzzle.
+./driver.sh $YEAR $DAY $PUZZLE $INPUT
+```
+
+## Building the puzzles
+
+The build system uses CMake.
+
+```sh
+cmake -Bbuild -S.
+cmake --build build
+```
+
+Note that some of the puzzles require OpenSSL for the MD5 implementation.  On macOS this also requires you to pass an appropariate value for `OPENSSL_ROOT_DIR` to cmake.  On x86 this is `-DOPENSSL_ROOT_DIR=/usr/local/opt/openssl` and on AArch64 this is `-DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl`.
+
+## Sources
+
+All code is written in C++.
+
+### Directory Layout
+
+Each year of puzzles is in a self-contained directory `YEAR`.  
+
+Within the directory there are the following files:
+
+ * `puzzle-DAY-NUMBER.cc`.  The source for puzzle NUMBER on day DAY in the year.
+ * `driver-DAY-NUMBER.sh`.  If present this is used as the driver to run that puzzle.
+ * `puzzle-DAY-NUMBER.CMakeLists.txt`.  If present this is included by `CMakeLists.txt`.  The variable `${puzzle_name}`
+   contains the name of the puzzle target.  Some puzzles use this to depend on OpenSSL. 
+
+### Command Line Interface
+
+The executable command line interface should just take the input in on standard-input and print its
+result on standard-output.
+
+If this is not possible the script `driver-DAY-NUMBER.sh` within the year directory should be provided.  It should take
+two arguments - the executable to run and the input file to use.