Document our usage of insta

Author: Kobzol

site/README.md

@@ -46,3 +46,8 @@ the `PORT` environment variable.
 ```console
 $ ./target/release/site <database>
 ```
+
+## Development
+We use [insta](https://github.com/mitsuhiko/insta) for snapshot testing. If any tests that use the
+`insta` macros fail, you should `cargo install cargo-insta` and then run `cargo insta review` to review
+the snapshot changes.