docs: document validateWebhook

[zeke]

Resolves #195

[aron]

Looks great, one minor comment to also document the fallback arguments for when the environment doesn't support the Request API.

[aron]

We should cover what to do if your environment doesn't support Request, in which case you pass through the parts manually:

const requestData = {
  id: "123",          // the `Webhook-Id` header
  timestamp: 0123456, // the `Webhook-Timestamp` header
  signature: "xyz",   // the `Webhook-Signature` header
  body: "",           // the request body as a string, ArrayBuffer or ReadableStream 
};
const webhookIsValid = await validateWebhook(requestData, secret);

[zeke]

Yeah good point. I opened an issue to follow up on that: #261