docs: add documentation for trembita usage on plugins.

README.md

@@ -34,6 +34,45 @@ yarn add trembita
 ## Usage
 Trembita is not supposed to be used directly, rather than to develop plugins.
 
+In order you can use a third party service by implementing your plugin, make sure you fill the next requeriments:
+
+-  Extend Trembita module.
+-  Construct the plugin by implementing the properties inherited from trembita module.
+-  Define the methods that contains the logic that expose third party library logic you want to use.
+
+One example of usage would be:
+
+```js
+const clientOptions = { 
+	headers: { 
+		header1: 'xxx', 
+		header2: 'yyy'
+	},
+	endpoint: 'http://serviceapi.com'
+};
+
+const MyAPIClient = class MyAPIClient extends Trembita {
+  constructor(options) {
+    super(...arguments);
+
+    this.getData = paramsQueryString => {
+      const params = {
+        url: `api/path/`,
+        qs: paramsQueryString,
+        expectedCodes: [200, 401, 403, 404],
+        headers: {
+          'header1':  this.header1,
+          'header2': this.header2
+        }
+      };
+      return this.request(params);
+    }
+  }
+}
+
+const client = new MyAPIClient(clientOptions);
+```
+
 ## Contribute
 
 See [the contribute file](CONTRIBUTING.md)!