Add problem spec README

problems/README.md

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+# Problem Generation
+
+This directory contains problem generators, which is in charge of generating
+problem inputs and their solutions. The de facto language for problem
+generators is Python.
+
+For an example of a problem generator, see [./01/problem.py](./01/problem.py).
+
+## Specification
+
+The server expects a problem generator to implement the following usages:
+
+- `$PROGRAM --seed $SEED`: generate a problem input using the given seed.
+- `$PROGRAM --seed $SEED --part1`: generate the part 1 solution using the given seed.
+- `$PROGRAM --seed $SEED --part2`: generate the part 2 solution using the given seed.
+
+Currently, only Python is supported as the language for problem generators.
+It would be trivial to support other languages, but it is not a priority at the
+moment.
+
+## Python
+
+Python problem generators must have a module path of `problems.NAME.problem`.
+For example, if the problem name is `01`, the module path must be
+`problems.01.problem`.
+
+To test this, simply run the following command:
+
+```sh
+python -m problems.01.problem --seed 0
+```