docs: add middleware docs

Author: janniks

packages/network/README.md

@@ -10,7 +10,7 @@ npm install @stacks/network
 
 ## Usage
 
-Creating a Stacks mainnet, testnet or mocknet network
+### Create a Stacks mainnet, testnet or mocknet network
 
 ```typescript
 import { StacksMainnet, StacksTestnet, StacksMocknet } from '@stacks/network';
@@ -22,19 +22,19 @@ const testnet = new StacksTestnet();
 const mocknet = new StacksMocknet();
 ```
 
-Setting a custom node URL
+### Set a custom node URL
 
 ```typescript
 const network = new StacksMainnet({ url: 'https://www.mystacksnode.com/' });
 ```
 
-Check if network is mainnet
+### Check if network is mainnet
 
 ```typescript
 const isMainnet = network.isMainnet();
 ```
 
-Example usage in transaction builder
+### Network usage in transaction building
 
 ```typescript
 import { makeSTXTokenTransfer } from '@stacks/transactions';
@@ -49,7 +49,29 @@ const txOptions = {
 const transaction = await makeSTXTokenTransfer(txOptions);
 ```
 
-Get various API URLs
+### Use the built-in API key middleware
+
+Some Stacks APIs make use API keys to provide less rate-limited plans.
+
+```typescript
+const myApiMiddleware = createApiKeyMiddleware('example_e8e044a3_41d8b0fe_3dd3988ef302');
+const myFetchFn = createFetchFn(apiMiddleware); // middlewares can be used to create a new fetch function
+const myMainnet = new StacksMainnet({ fetchFn: myFetchFn }); // the fetchFn options can be passed to a StacksNetwork to override the default fetch function
+
+const txOptions = {
+  recipient: 'SP3FGQ8Z7JY9BWYZ5WM53E0M9NK7WHJF0691NZ159',
+  amount: 12345n,
+  senderKey: 'b244296d5907de9864c0b0d51f98a13c52890be0404e83f273144cd5b9960eed01',
+  memo: 'some memo',
+  anchorMode: AnchorMode.Any,
+  network: myMainnet, // make sure to pass in the custom network object
+};
+const transaction = await makeSTXTokenTransfer(txOptions); // fee-estimation will use the custom fetchFn
+
+const response = await broadcastResponse(transaction, myMainnet); // make sure to broadcast via the custom network object
+```
+
+### Get various API URLs
 
 ```typescript
 const txBroadcastUrl = network.getBroadcastApiUrl();