Add docs for newly exposed API methods

docs/src/deriveKeypair.md.ejs

@@ -0,0 +1,21 @@
+## deriveKeypair
+
+`deriveKeypair(seed: string) : {privateKey: string, publicKey: string}`
+
+Derive a public and private key from a seed.
+
+### Parameters
+
+This method takes one parameter, the seed from which to derive the public and private key.
+
+### Return Value
+
+This method returns an object containing the public and private components of the keypair corresponding to the seed.
+
+### Example
+
+```javascript
+var keypair = api.deriveKeypair(seed)
+var public_key = keypair.publicKey;
+var private_key = keypair.privateKey;
+```