docs: Update README with general description and examples (#3302)

Author: iker-barriocanal

packages/nextjs/README.md

@@ -7,13 +7,54 @@
 
 # Official Sentry SDK for NextJS
 
-TODO: npm version, npm dm, npm dt, typedoc
-
 ## Links
 
 - [Official SDK Docs](https://docs.sentry.io/quickstart/)
 - [TypeDoc](http://getsentry.github.io/sentry-javascript/)
 
-## Usage
+## General
+
+This package is a wrapper around `@sentry/node` for the server and `@sentry/react` for the client, with added functionality related to NextJS.
+
+To use this SDK, call `Sentry.init(options)` as early as possible in the server and the client;
+this will initialize the SDK and hook into the environment.
+
+
+```javascript
+import * as Sentry from '@sentry/nextjs';
+
+Sentry.init({
+  dsn: '__DSN__',
+  // ...
+});
+```
+
+To set context information or send manual events, use the exported functions of `@sentry/minimal`.
+Note that these functions will not perform any action before you have called `Sentry.init()`:
+
+```javascript
+import * as Sentry from '@sentry/nextjs';
+
+// Set user information, as well as tags and further extras
+Sentry.configureScope(scope => {
+  scope.setExtra('battery', 0.7);
+  scope.setTag('user_mode', 'admin');
+  scope.setUser({ id: '4711' });
+  // scope.clear();
+});
+
+// Add a breadcrumb for future events
+Sentry.addBreadcrumb({
+  message: 'My Breadcrumb',
+  // ...
+});
 
-TODO
+// Capture exceptions, messages or manual events
+Sentry.captureMessage('Hello, world!');
+Sentry.captureException(new Error('Good bye'));
+Sentry.captureEvent({
+  message: 'Manual',
+  stacktrace: [
+    // ...
+  ],
+});```