Add README with environment info

README.md

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+# Image API
+This is an Actix-web server for serving images and videos from a filesystem.
+Upon first run it will generate thumbnails for all images and videos at `DEFAULT_PATH`.
+
+## Environment
+There are a handful of required environment variables to have the API run.
+They should be defined where the binary is located or above it in an `.env` file.
+
+- `DATABASE_URL` is a path or url to a database (currently only SQLite is tested)
+- `BASE_PATH` is the root from which you want to serve images and videos
+- `THUMBNAILS` is a path where generated thumbnails should be stored
+- `BIND_URL` is the url and port to bind to (typically your own IP address)
+- `SECRET_KEY` is the *hopefully* random string to sign Tokens with
+