Document that start-web-server needs you to run migrations first

Author: jyn514

README.md

@@ -64,10 +64,13 @@ cargo build
 # Start the extenal services
 docker-compose up -d db s3
 # Build a sample crate to make sure it works
-# This sets up the docs.rs build environment, including installing the nightly
-# Rust toolchain. This will take a while the first time but will be cached afterwards.
+# This sets up the docs.rs build environment,
+# including running the migrations and installing the nightly Rust toolchain.
+# This will take a while the first time but will be cached afterwards.
 cargo run -- build crate regex 1.3.1
 # This starts the web server but does not build any crates.
+# It does not automatically run the migrations, so you need to do that manually.
+cargo run database migrate
 cargo run start-web-server
 ```