make libsecp256k1 available with a build tag.

README.md

@@ -126,6 +126,25 @@ nostr.InfoLogger = log.New(io.Discard, "", 0)
 go run example/example.go
 ```
 
+### Using [`libsecp256k1`](https://github.com/bitcoin-core/secp256k1)
+
+[`libsecp256k1`](https://github.com/bitcoin-core/secp256k1) is very fast:
+
+```
+goos: linux
+goarch: amd64
+cpu: Intel(R) Core(TM) i5-2400 CPU @ 3.10GHz
+BenchmarkWithoutLibsecp256k1/sign-4          	    2794	    434114 ns/op
+BenchmarkWithoutLibsecp256k1/check-4         	    4352	    297416 ns/op
+BenchmarkWithLibsecp256k1/sign-4             	   12559	     94607 ns/op
+BenchmarkWithLibsecp256k1/check-4            	   13761	     84595 ns/op
+PASS
+```
+
+But to use it you need the host to have it installed as a shared library and CGO to be supported, so we don't compile against it by default.
+
+To use it, use `-tags=libsecp256k1` whenever you're compiling your program that uses this library.
+
 ## Warning: risk of goroutine bloat (if used incorrectly)
 
 Remember to cancel subscriptions, either by calling `.Unsub()` on them or ensuring their `context.Context` will be canceled at some point.