Clarify hdkey

Author: paulmillr

README.md

@@ -205,7 +205,26 @@ Note: if you've been using ethereum-cryptography v0.1, it had different API. We'
 
 ## BIP32 HD Keygen
 
-This module exports a single class whose type is
+This module exports a single class `HDKey`, which should be used like this:
+
+```ts
+const { HDKey } = require("ethereum-cryptography/secp256k1");
+const hdkey1 = HDKey.fromMasterSeed(seed);
+const hdkey2 = HDKey.fromExtendedKey(base58key);
+const hdkey3 = HDKey.fromJSON({ xpriv: string });
+
+// props
+[hdkey1.depth, hdkey1.index, hdkey1.chainCode];
+console.log(hdkey2.privateKey, hdkey2.publicKey);
+console.log(hdkey3.derive("m/0/2147483647'/1"));
+const sig = hdkey3.sign(hash);
+hdkey3.verify(hash, sig);
+```
+
+Note: do not set `privateKey` and `publicKey` directly. Use `wipePrivateData()` to
+clean-up the instance.
+
+The full API is:
 
 ```ts
 class HDKey {