Add flow type signatures.

.flowconfig

@@ -0,0 +1,8 @@
+[ignore]
+.*/node_modules/documentation/*
+
+[libs]
+
+[include]
+
+[options]

package.json

@@ -42,7 +42,8 @@
     "async": "^2.6.0",
     "libp2p-crypto": "~0.11.0",
     "lodash": "^4.17.4",
-    "multihashes": "~0.4.12"
+    "multihashes": "~0.4.12",
+    "callback.flow": "1.0.0"
   },
   "repository": {
     "type": "git",

src/index.js.flow

@@ -0,0 +1,44 @@
+// @flow
+
+import type { Callback } from "callback.flow"
+import * as crypto from "libp2p-crypto"
+
+const { RsaPublicKey, RsaPrivateKey } = crypto.key.rsa
+
+export interface JSONPeerId {
+  id: string;
+  privKey: string;
+  pubKey: string;
+}
+
+export interface PeerId {
+  id: Buffer;
+  privKey: RsaPrivateKey;
+  pubKey: RsaPublicKey;
+
+  toJSON(): JSONPeerId;
+  toPrint(): JSONPeerId;
+  toB58String(): string;
+  toBytes(): Buffer;
+  toHexString(): string;
+  isEqual(PeerId): boolean;
+  marshalPubKey(): Buffer;
+  marshalPrivKey(): Buffer;
+  isValid(Callback<Error>): void;
+}
+
+export type CreateOptions = {
+  bits?: number
+}
+
+declare export default {
+  create(CreateOptions, Callback<Error, PeerId>): void,
+  create(Callback<Error, PeerId>): void,
+  createFromHexString(string): PeerId,
+  createFromBytes(Buffer): PeerId,
+  createFromB58String(string): PeerId,
+  createFromPubKey(string | Buffer, Callback<Error, PeerId>): void,
+  createFromPrivKey(string | Buffer, Callback<Error, PeerId>): void,
+  createFromJSON(JSONPeerId, Callback<Error, PeerId>): void,
+  isPeerId(mixed): boolean
+}