add template for ocaml

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+# Advent of code with OCaml
+
+## Getting started
+
+1. Create a repository using this template
+1. Install dependencies via `opam install . --deps-only`
+1. Put your session cookie into `.session`
+1. (Optional) Put the year into `.year` (e.g. 2022)
+
+## HowTo
+
+For each day, first specify a type `t` for the parsed input, and then
+there are 3 functions to implement:
+
+- `input : string -> t`, a function to parse the input into a user specified
+    type `t`
+- `part1 : t -> unit` and `part2 : t -> unit`, functions to execute both parts
+
+The functor `Day.Make` generates a function `run` to run the pipeline.
+
+```ocaml
+    val run : ?only_part1:bool -> ?only_part2:bool -> string -> unit
+```
+
+The main executable, `aoc`, can be invoked via
+
+```bash
+    dune exec aoc DAY
+```
+
+which will invoke the `run` function for that DAY, with input taken from
+`inputs` directory.
+If no input file is present, then the input file would be downloaded using your
+session cookie.
+
+If you want to test your program with the small example for the day, the
+easiest way is to put it the `example` in the template file.
+The expectation test can be invoked with `dune test`, and updated with `dune
+promote`, see more in [dune documentation on expectation
+tests](https://dune.readthedocs.io/en/stable/tests.html#inline-expectation-tests).