📝 Document matcher caveats in the README

State the end-user limits once, in README § Caveats: a member colliding
with its stringification, the unsupported symbol/bigint, and NaN/-0. Drop
the duplicated known-issue prose from development/library.md and the
source comment; link to the README instead.

README.md

@@ -9,7 +9,7 @@ ordinary objects whose `matches` method is a TypeScript type guard, so narrowing
 composes the way any other guard does. It is deliberately not a regex engine and
 not a macro: there is no transpiler and no DSL to learn, and the type-level
 contract is the feature — see [development/library.md](./development/library.md)
-for the design decisions and the known limitations.
+for the design decisions and [Caveats](#caveats) for the limits.
 
 ## Requirements
 
@@ -23,6 +23,18 @@ for the design decisions and the known limitations.
 
 Yet to be implemented
 
+## Caveats
+
+- **A value and its stringification collide.** Object keys stringify, so a
+  universe that mixes a member with the string it stringifies to — `1 | "1"`,
+  `true | "true"`, `null | "null"` — collapses to a single handler key and both
+  members are routed to it. Use one form or the other.
+- **`symbol` and `bigint` are not supported.** A `symbol` brand is a
+  compile-time phantom with nothing to match at runtime, and a `bigint` is not a
+  valid property key; neither satisfies the matcher's universe constraint.
+- **`NaN` and `-0` cannot be matched specifically.** They have no literal type,
+  so both stay part of `number`.
+
 ## License
 
 MIT © 2025 tmu. See [LICENSE](./LICENSE).