docs: document customizing fetch (#276)

README.md

@@ -250,6 +250,45 @@ console.log(raw.headers.get('X-My-Header'));
 console.log(externalAccount.id);
 ```
 
+## Customizing the fetch client
+
+By default, this library uses `node-fetch` in Node, and expects a global `fetch` function in other environments.
+
+If you would prefer to use a global, web-standards-compliant `fetch` function even in a Node environment,
+(for example, if you are running Node with `--experimental-fetch` or using NextJS which polyfills with `undici`),
+add the following import before your first import `from "ModernTreasury"`:
+
+<!-- prettier-ignore -->
+```ts
+// Tell TypeScript and the package to use the global web fetch instead of node-fetch.
+// Note, despite the name, this does not add any polyfills, but expects them to be provided if needed.
+import "modern-treasury/shims/web";
+import ModernTreasury from "modern-treasury";
+```
+
+To do the inverse, add `import "modern-treasury/shims/node"` (which does import polyfills).
+This can also be useful if you are getting the wrong TypeScript types for `Response` - more details [here](https://github.com/Modern-Treasury/modern-treasury-node/src/_shims#readme).
+
+You may also provide a custom `fetch` function when instantiating the client,
+which can be used to inspect or alter the `Request` or `Response` before/after each request:
+
+```ts
+import { fetch } from 'undici'; // as one example
+import ModernTreasury from 'modern-treasury';
+
+const client = new ModernTreasury({
+  fetch: (url: RequestInfo, init?: RequestInfo): Response => {
+    console.log('About to make request', url, init);
+    const response = await fetch(url, init);
+    console.log('Got response', response);
+    return response;
+  },
+});
+```
+
+Note that if given a `DEBUG=true` environment variable, this library will log all requests and responses automatically.
+This is intended for debugging purposes only and may change in the future without notice.
+
 ## Configuring an HTTP(S) Agent (e.g., for proxies)
 
 By default, this library uses a stable agent for all http/https requests to reuse TCP connections, eliminating many TCP & TLS handshakes and shaving around 100ms off most requests.