docs: governance canister local snippet (#487)

# Motivation

I followed the README to setup a sample script and I have to say that an
example of local usages would have been useful.
Therefore providing this PR to add it.

packages/nns/README.md

@@ -45,6 +45,27 @@ const { listNeurons } = GovernanceCanister.create({
 const myNeurons = await listNeurons({ certified: false });
 ```
 
+To execute this on a local environment, you will need to fetch the root key when initializing the agent. Additionally, you might need to adapt the port. The following snippet also demonstrates how you can inline a canister ID as well.
+
+```typescript
+import { GovernanceCanister } from "@dfinity/nns";
+import { createAgent } from "@dfinity/utils";
+import { Principal } from "@dfinity/principal";
+
+const agent = await createAgent({
+  identity,
+  host: "http://localhost:8000",
+  fetchRootKey: true,
+});
+
+const { listNeurons } = GovernanceCanister.create({
+  agent,
+  canisterId: Principal.fromText("rrkah-fqaaa-aaaaa-aaaaq-cai"),
+});
+
+const myNeurons = await listNeurons({ certified: false });
+```
+
 ## Features
 
 `nns-js` implements following features: