chore(docs:postgres): add usage instructions (#539)

* chore(docs:postgres): add usage instructions

* docs: add pg-pool install

* docs: add pg-pool version

Co-authored-by: Valentin Marchaud <thisismac47@gmail.com>

packages/opentelemetry-plugin-postgres/opentelemetry-plugin-pg/README.md

@@ -13,19 +13,62 @@ For automatic instrumentation see the
 
 ```bash
 npm install --save @opentelemetry/plugin-pg
+npm install --save @opentelemetry/plugin-pg-pool
 ```
 
 ## Usage
 
+To load all of the [default supported plugins](https://github.com/open-telemetry/opentelemetry-js#plugins), use the below approach. Each plugin is only loaded when the module that it patches is loaded; in other words, there is no  computational overhead for listing plugsin for unused modules.
+
+```js
+const { NodeTracer } = require('@opentelemetry/node');
+
+const tracer = new NodeTracer(); // All default plugins will be used
+```
+
+If instead you would just load a specific plugin (**pg** in this case), specify it in the `NodeTracer` configuration.
+
 ```js
-const opentelemetry = require('@opentelemetry/plugin-pg');
+const { NodeTracer } = require('@opentelemetry/node');
 
-// TODO: DEMONSTRATE API
+const tracer = new NodeTracer({
+  plugins: {
+    pg: {
+      enabled: true,
+      // You may use a package name or absolute path to the module
+      path: '@opentelemetry/plugin-pg',
+    }
+  }
+});
 ```
 
+If you are using any of the [`pg.Pool`](https://node-postgres.com/api/pool) APIs, you will also need to include the [`pg-pool` plugin](../opentelemetry-plugin-pg-pool).
+
+```js
+const { NodeTracer } = require('@opentelemetry/node');
+
+const tracer = new NodeTracer({
+  plugins: {
+    pg: {
+      enabled: true,
+      // You may use a package name or absolute path to the module
+      path: '@opentelemetry/plugin-pg',
+    },
+    'pg-pool': {
+      enabled: true,
+      // You may use a package name or absolute path to the module
+      path: '@opentelemetry/plugin-pg-pool',
+    },
+  }
+});
+```
+
+See [examples/postgres](https://github.com/open-telemetry/opentelemetry-js/tree/master/examples/postgres) for a short example.
+
 ## Supported Versions
 
 - [pg](https://npmjs.com/package/pg): `7.x`
+- [pg-pool](https://npmjs.com/package/pg-pool): `2.x` (Installed by `pg`)
 
 ## Useful links
 - For more information on OpenTelemetry, visit: <https://opentelemetry.io/>