allow using libsecp256k1 for signature verification in subscriptions.

libsecp256k1/README.md

@@ -1,4 +1,8 @@
-This is faster than the pure Go version:
+This wraps [libsecp256k1](https://github.com/bitcoin-core/secp256k1) with `cgo`.
+
+It doesn't embed the library or anything smart like that because I don't know how to do it, so you must have it installed in your system.
+
+It is faster than the pure Go version:
 
 ```
 goos: linux
@@ -8,3 +12,22 @@ cpu: AMD Ryzen 3 3200G with Radeon Vega Graphics
 BenchmarkSignatureVerification/btcec-4         	     145	   7873130 ns/op	  127069 B/op	     579 allocs/op
 BenchmarkSignatureVerification/libsecp256k1-4  	     502	   2314573 ns/op	  112241 B/op	     392 allocs/op
 ```
+
+To use it manually, just import. To use it inside the automatic verification that happens for subscriptions, set it up with a `SimplePool`:
+
+```go
+pool := nostr.NewSimplePool()
+pool.SignatureChecker = func (evt nostr.Event) bool {
+	ok, _ := libsecp256k1.CheckSignature(evt)
+	return ok
+}
+```
+
+Or directly to the `Relay`:
+
+```go
+relay := nostr.RelayConnect(context.Background(), "wss://relay.nostr.com", nostr.WithSignatureChecker(func (evt nostr.Event) bool {
+	ok, _ := libsecp256k1.CheckSignature(evt)
+	return ok
+}))
+```