Add not about running tests to expression docs

Author: bkeepers

README.md

@@ -11,6 +11,8 @@ To learn more about JSON Schema, read [Understanding JSON Schema](https://json-s
 1. Describe arguments by creating a new file in [`schemas/`](schemas/) named `NewName.schema.json`. You can copy an existing function that has similar semantics to get started.
 2. Add the new function in [`schemas/schema.json`](schemas/schema.json) to `$defs/function`.
 3. Create a new file in [`examples/`](./examples) named `NewName.json` with valid and invalid examples for the new function. See other examples for inspiration.
-4. Implement the function in [`lib/flipper/expressions/`](../../lib/flipper/expressions/).
+4. Run `yarn test` in `packages/expressions` and ensure tests pass.
+5. Implement the function in [`lib/flipper/expressions/`](../../lib/flipper/expressions/).
+6. Run `rspec` to ensure tests pass.
 
 See [this commit that adds Min/Max functions](https://github.com/jnunemaker/flipper/commit/ee46fab0cda21a32c3a921a8ed1fb94b0842b6b4) for a concrete example.