docs(node-sdk): extend README (open-telemetry#367)

Author: danielkhan

packages/opentelemetry-node-sdk/README.md

@@ -4,28 +4,96 @@
 [![devDependencies][devDependencies-image]][devDependencies-url]
 [![Apache License][license-image]][license-image]
 
-This module provides automatic tracing for Node.js applications.
+This module provides *automated instrumentation and tracing* for Node.js applications.
 
 For manual instrumentation see the
 [@opentelemetry/basic-tracer](https://github.com/open-telemetry/opentelemetry-js/tree/master/packages/opentelemetry-basic-tracer) package.
 
-## How does automatic tracing work?
-> TODO: description
+## How does automated instrumentation work?
+node-sdk exposes a `NodeTracer` that will automatically hook into the module loader of Node.js.
+
+For this to work, please make sure that `NodeTracer` is initialized before any other module of your application, (like `http` or `express`) is loaded.
+
+OpenTelemetry comes with a growing number of instrumentation plugins for well know modules (see [supported modules](https://github.com/open-telemetry/opentelemetry-js#plugins)) and an API to create custom plugins (see [the plugin developer guide](https://github.com/open-telemetry/opentelemetry-js/blob/master/doc/plugin-guide.md)).
+
+
+Whenever a module is loaded `NodeTracer` will check if a matching instrumentation plugin has been installed.
+
+> **Please note:** This module does *not* bundle any plugins. They need to be installed separately.
+
+If the respective plugin was found, it will be used to patch the original module to add instrumentation code.
+This is done by wrapping all tracing-relevant functions.
+
+This instrumentation code will automatically
+- extract a trace-context identifier from inbound requests to allow distributed tracing (if applicable)
+- make sure that this current trace-context is propagated while the transaction traverses an application (see [@opentelemetry/opentelemetry-scope-base](https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-scope-base/README.md) for an in-depth explanation)
+- add this trace-context identifier to outbound requests to allow continuing the distributed trace on the next hop (if applicable)
+- create and end spans
+
+In short, this means that this module will use provided plugins to automatically instrument your application to produce spans and provide end-to-end tracing by just adding a few lines of code.
+
+## Creating custom spans on top of auto-instrumentation
+Additionally to automated instrumentation, `NodeTracer` exposes the same API as [@opentelemetry/basic-tracer](https://github.com/open-telemetry/opentelemetry-js/tree/master/packages/opentelemetry-basic-tracer), allowing creating custom spans if needed.
 
 ## Installation
 
 ```bash
+npm install --save @opentelemetry/core
 npm install --save @opentelemetry/node-sdk
+
+# Install instrumentation plugins
+npm install --save @opentelemetry/plugin-http
+npm install --save @opentelemetry/plugin-grpc
 ```
 
 ## Usage
 
+The following code will configure the `NodeTracer` to instrument `http` using `@opentelemetry/plugin-http`.
+
 ```js
-const opentelemetry = require('@opentelemetry/node-sdk');
+const opentelemetry = require('@opentelemetry/core');
+const { NodeTracer } = require('@opentelemetry/node-sdk');
 
-// TODO: DEMONSTRATE API
+// Create and configure NodeTracer
+const tracer = new NodeTracer({
+  plugins: {
+    http: {
+      enabled: true,
+      // You may use a package name or absolute path to the file.
+      path: '@opentelemetry/plugin-http',
+      // http plugin options
+    }
+  }
+});
+
+// Initialize the tracer
+opentelemetry.initGlobalTracer(tracer);
+
+// Your application code - http will automatically be instrumented if
+// @opentelemetry/plugin-http is present
+const http = require('http');
 ```
 
+To enable instrumentation for all [supported modules](https://github.com/open-telemetry/opentelemetry-js#plugins), create an instance of `NodeTracer` without providing any plugin configuration to the constructor.
+
+```js
+const opentelemetry = require('@opentelemetry/core');
+const { NodeTracer } = require('@opentelemetry/node-sdk');
+
+// Create and initialize NodeTracer
+const tracer = new NodeTracer();
+
+// Initialize the tracer
+opentelemetry.initGlobalTracer(tracer);
+
+// Your application code
+// ...
+```
+
+## Examples
+See how to automatically instrument [http](https://github.com/open-telemetry/opentelemetry-js/tree/master/examples/http) and [gRPC](https://github.com/open-telemetry/opentelemetry-js/tree/master/examples/grpc) using node-sdk.
+
+
 ## Useful links
 - For more information on OpenTelemetry, visit: <https://opentelemetry.io/>
 - For more about OpenTelemetry JavaScript: <https://github.com/open-telemetry/opentelemetry-js>