Document new SCM plugin system

docs/_includes/navigation.html

@@ -46,6 +46,7 @@
   <li><a href="/documentation/advanced-features/ssh-kit">Remote Commands with SSHKit</a></li>
   <li><a href="/documentation/advanced-features/ignoring">Preventing file deployment with gitattributes</a></li>
   <li><a href="/documentation/advanced-features/validation-of-variables">Validation of variables</a></li>
+  <li><a href="/documentation/advanced-features/custom-scm">Custom SCM</a></li>
   <li class="divider"></li>
 
   <h5>Plugins</h5>

docs/documentation/advanced-features/custom-scm/index.markdown

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+---
+title: Custom SCM
+layout: default
+---
+
+Capistrano uses what it calls "SCM plugins" (Source Code Management), to deploy
+your source code from a central repository. Out of the box, Capistrano has three
+plugins to handle Git, Subversion, and Mercurial repositories.
+
+Most Capistrano users are well-served by these default implementations. To
+choose an SCM, users add it to their Capfile, like this:
+
+```ruby
+require "capistrano/scm/git"
+install_plugin Capistrano::SCM::Git
+```
+
+It is also possible to provide a custom SCM plugin, in order to change how
+Capistrano checks out your application's source code. SCM plugins can be
+packaged as Ruby gems and distributed to other users.
+
+This document is a short guide to writing your own plugin. *It applies to
+Capistrano 3.5.0 and newer.*
+
+### 1. Write a Ruby class that extends Capistrano::Plugin
+
+Let's say you want to create a "Foo" SCM. You'll need to write a plugin class,
+like this:
+
+```ruby
+require "capistrano/plugin"
+
+# By convention, Capistrano plugins are placed in the
+# Capistrano namespace. This is completely optional.
+module Capistrano
+  class FooPlugin < ::Capistrano::Plugin
+    def set_defaults
+      # Define any variables needed to configure the plugin.
+      # set_if_empty :myvar, "my-default-value"
+    end
+  end
+end
+```
+
+### 2. Implement the create_release task
+
+When the user runs `cap deploy`, your SCM is responsible for creating the
+release directory and copying the application source code into it. You need to
+do this using a `create_release` task that is namespaced to your plugin.
+
+Inside your plugin class, use the `define_tasks` and `register_hooks` methods
+like this:
+
+```ruby
+def define_tasks
+  # The namespace can be whatever you want, but its best
+  # to choose a name that matches your plugin name.
+  namespace :foo do
+    # The task *must* be named `create_release`
+    task :create_release do
+      # Your code to create the release directory and copy
+      # the source code into it goes here.
+      on release_roles :all do
+        execute :mkdir, "-p", release_path
+        # ...
+      end
+    end
+  end
+end
+
+def register_hooks
+  # Tell Capistrano to run the custom create_release task
+  # during deploy.
+  after "deploy:new_release_path", "foo:create_release"
+end
+```
+
+### 3. Implement the set_current_revision task
+
+Similar to how you defined a `create_release`, you should also define a
+`set_current_revision` task. The purpose of this task is to set a special
+variable that Capistrano uses to write to the deployment log.
+
+```ruby
+# Your task should do something like this
+set :current_revision, "..."
+
+# Register this hook to ensure your task runs
+before "deploy:set_current_revision", "foo:set_current_revision"
+```
+
+### 4. Use the plugin
+
+To use your plugin, simply `require` the file where your plugin class is
+defined, and then use `install_plugin`.
+
+```ruby
+# In Capfile
+require_relative "path/to/foo_plugin.rb"
+install_plugin Capistrano::FooPlugin
+```
+
+That's it!
+
+### 5. Distribute your plugin as a gem
+
+Packaging and distributing Ruby gems is outside the scope of this document.
+However, there is nothing Capistrano-specific that needs to be done here; just
+create a standard gem that contains your plugin class.
+
+Users can then install your plugin by adding its gem to their Gemfile:
+
+```ruby
+gem "your-gem-name", :group => :development
+```
+
+And then add it the Capfile:
+
+```ruby
+require "your-gem-name"
+install_plugin YourPluginClass
+```
+
+### 6. Getting help
+
+For more techniques and ideas, check out the implementations of the default Git,
+Subversion, and Mercurial plugins in the official
+[Capistrano repository](https://github.com/capistrano/capistrano) on GitHub.
+All three follow the same patterns described in this document.
+
+Otherwise open a [GitHub issue](https://github.com/capistrano/capistrano/issues)
+with your questions or feedback. Thanks!