docs: add README

Author: danieljtrowbridge

README.md

@@ -0,0 +1,12 @@
+# Evaluation Function Testing Server
+
+This server is intended to be run locally while using the Response Area
+Sandbox. It acts as a proxy between the Sandbox (running within a web browser)
+and an evaluation function.
+
+Web browsers send a [preflight
+request](https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request)
+before every GET or POST request which uses the OPTIONS verb. Evaluation
+functions (running locally or on AWS) are not configured to respond to OPTIONS
+requests, so this proxy is responsible for replying to the preflight request and
+forwarding just the POST request to the evaluation function.