Add a "code style" section to the readme

README.md

@@ -14,3 +14,15 @@ To run type checks, run `deno task typecheck`.
 
 To run all checks (tests, linting, code formatting, and type checks), run
 `deno task check-all`.
+
+## Code style
+
+- Make sure all checks pass with `deno task check-all`.
+- Prefer arrow functions.
+- `throw` should be used when the developer makes a mistake, not for "expected"
+  errors. For example, if someone uploads a bogus file, that shouldn't throw an
+  error.
+- Bad news first:
+  - If a function returns `null` if an error case, define the type
+    `null | Result`, not `Result | type`.
+  - Test error cases first.