Update documentation (#127)

Added extending multiple plugins
Added creating plugin

packages/docs/src/docs/configuration.md

@@ -5,20 +5,6 @@ Boll is configured by way of the `.boll.config.js` file in the root of a given p
 Each boll configuration must load rules to be run across the repository. It may either
 extend an existing configuration or fabricate a new configuration.
 
-## Extending an existing configuration
-
-To extend an configuration, first make sure rules are loaded, then export an object with
-an `extends` key.
-
-```js
-"use strict";
-const { bootstrapRecommendedConfiguration } = require('@boll/recommended');
-bootstrapRecommendedConfiguration();
-module.exports = {
-    extends: "boll:recommended"
-};
-```
-
 ## Creating a new configuration
 
 A configuration is a list of `RuleSet` objects. Each `RuleSet` must define a `fileLocator`
@@ -42,4 +28,61 @@ module.exports = {
 };
 ```
 
-`@boll/core` provides several `fileLocator` implementations (see [FileGlob](../api/core/interfaces/fileglob)) out of the box.
+`@boll/core` provides several `fileLocator` implementations (see [FileGlob](../api/core/interfaces/fileglob)) out of the box.
+
+
+[Learn how to create rules](custom-rule)
+
+## Extending an existing configuration
+
+To extend an configuration, install the plugin and export an object with
+an `extends` key.
+
+```js
+"use strict";
+
+module.exports = {
+    extends: "boll:recommended"
+};
+```
+
+You may also extend from multiple plugins.
+```js
+"use strict";
+
+module.exports = {
+    extends: ["boll:recommended","plugin:check-readme"]
+};
+```
+
+## Creating a plugin
+A plugin is a configuration that can be extended from to provide additional rules. To create a plugin,
+create a module which exports a `bootstrap` function. The plugin's configuration name also has to begin with the prefix `plugin:`.
+
+```js
+"use strict";
+const { addRule, WorkspacesGlob, ConfigRegistryInstance } = require("@boll/core");
+// a custom created rule to check readme file
+const { ensureReadMe } = require("./rules/readme");
+
+const readMeConfig = {
+  name: "plugin:check-readme",
+  ruleSets: [
+    {
+      fileLocator: new WorkspacesGlob(),
+      checks: {
+        file: [{ rule: "ensureReadMe" }]
+      }
+    }
+  ]
+};
+
+function bootstrap() {
+    addRule(ensureReadme);
+    ConfigRegistryInstance.register(readMeConfig);
+}
+
+module.exports = {
+    bootstrap
+};
+```