add evaluation description

Evaluation.md

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+## Evaluating models and prompts
+
+Evaluation dataset consists of 16 screenshots. A Python script for running screenshot-to-code on the dataset and a UI for rating outputs is included. With this set up, we can compare and evaluate various models and prompts.
+
+### Running evals
+
+- Input screenshots should be located at `backend/evals_data/inputs` and the outputs will be `backend/evals_data/outputs`. If you want to modify this, modify `EVALS_DIR` in `backend/evals/config.py`. You can download the input screenshot dataset here: TODO.
+- Set a stack (`STACK` var) in `backend/run_evals.py`
+- Run `python backend/run_evals.py` - this runs the screenshot-to-code on the input dataset in parallel but it will still take a few minutes to complete.
+- Once the script is done, you can find the outputs in `backend/evals_data/outputs`.
+
+### Rating evals
+
+In order to view and rate the outputs, visit your front-end at `/evals`.
+
+- Rate each output on a scale of 1-4
+- You can also print the page as PDF to share your results with others.
+
+Generally, I run three tests for each model/prompt + stack combo and take the average score out of those tests to evaluate.