Add ability to update snapshots (#4)

README.md

@@ -5,8 +5,9 @@ An [expect-testing](https://blog.janestreet.com/the-joy-of-expect-tests/) librar
 
 # Current status
 
-Basic mechanism works, but I haven't yet decided how the ergonomic updating of the input text will work.
-Ideally it would edit the input AST, but I don't yet know if that's viable.
+The basic mechanism works.
+Snapshot updating is vibe-coded with Opus 4 and is purely text-based; I didn't want to use the F# compiler services because that's a pretty heavyweight dependency which should be confined to a separate test runner entity.
+It's not very well tested, and I expect it to be kind of brittle.
 
 # How to use
 
@@ -35,6 +36,10 @@ let ``This test fails: plain text comparison of ToString`` () =
     }
 ```
 
+# Limitations
+
+* The snapshot updating mechanism *requires* you to use verbatim string literals. While the test assertions will work correctly if you do `snapshot ("foo" + "bar" + f 3)`, for example, the updating code is liable to do something undefined in that case. Also do not use format strings (`$"blah"`).
+
 # Licence
 
 MIT.