Docs: Add gulp.registry API & examples

Author: SyntaxRules

docs/API.md

@@ -682,6 +682,59 @@ gulp.tree({ deep: true })
 */
 ```
 
+### gulp.registry([registry])
+
+Get or set the underlying task registry. Inherited from [undertaker]; see the undertaker documention on [registries](https://github.com/phated/undertaker#registryregistryinstance). Using this, you can change registries that enhance gulp in different ways. Utilizing a custom registry has at least three use cases:
+
+- [Sharing tasks](https://github.com/phated/undertaker#sharing-tasks)
+- [Sharing functionality](https://github.com/phated/undertaker#sharing-functionalities). (e.g. you could override the task prototype to add some additional logging, bind task metadata or include some config settings.)
+- Handling other behavior that hooks into the registry lifecycle (see [gulp-hub](https://github.com/frankwallis/gulp-hub) for an example)
+
+To build your own custom registry see the [undertaker documentation on custom registries](https://github.com/phated/undertaker#custom-registries).
+
+#### registry
+
+A registry instance or constructor. When passed in, the tasks from the current registry will be transferred to the new registry and then current registry will be replaced with the new registry.
+
+#### Example
+
+This example shows how to create and use a simple custom registry to add tasks.
+
+```js
+//gulpfile.js
+var gulp = require('gulp');
+
+var companyTasks = require('./myCompanyTasksRegistry.js');
+
+gulp.registry(companyTasks);
+
+gulp.task('one', gulp.parallel('someCompanyTask', function(done) {
+  console.log('in task one');
+  done();
+}));
+```
+
+```js
+//myCompanyTasksRegistry.js
+var util = require('util');
+
+var DefaultRegistry = require('undertaker-registry');
+
+function MyCompanyTasksRegistry() {
+  DefaultRegistry.call(this);
+
+  this.set('clean', function(done) {
+    done();
+  });
+  this.set('someCompanyTask', function(done) {
+    console.log('performing some company task.');
+    done();
+  });
+}
+util.inherits(MyCompanyTasksRegistry, DefaultRegistry);
+
+module.exports = new MyCompanyTasksRegistry();
+```
 
 [Function.name]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name
 [gaze]: https://github.com/shama/gaze