add Flow page under getting-started section

documentation/getting-started/flow/index.markdown

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+---
+title: Flow
+layout: default
+---
+
+Capistrano v3 provides a default **deploy flow** and a **rollback flow**:
+
+### Deploy flow
+
+When you run `cap production deploy`, it invokes the following tasks in
+sequence:
+
+```
+deploy:starting    - start a deployment, make sure everything is ready
+deploy:started     - started hook (for custom tasks)
+deploy:updating    - update server(s) with a new release
+deploy:updated     - updated hook
+deploy:publishing  - publish the new release
+deploy:published   - published hook
+deploy:finishing   - finish the deployment, clean up everything
+deploy:finished    - finished hook
+```
+
+Notice there are several hook tasks e.g. `:started`, `:updated` for
+you to hook up custom tasks into the flow using `after()` and `before()`.
+
+### Rollback flow
+
+When you run `cap production deploy:rollback`, it invokes the following
+tasks in sequence:
+
+```
+deploy:starting
+deploy:started
+deploy:reverting           - revert server(s) to previous release
+deploy:reverted            - reverted hook
+deploy:publishing
+deploy:published
+deploy:finishing_rollback  - finish the rollback, clean up everything
+deploy:finished
+```
+
+As you can see, rollback flow shares many tasks with deploy flow. But note that, rollback flow runs its own `:finishing_rollback` task because its
+cleanup process is usually different from deploy flow.
+
+### Flow examples
+
+Assume you require the following files in `Capfile`,
+
+```
+# Capfile
+require 'capistrano/setup'
+require 'capistrano/deploy'
+require 'capistrano/bundler'
+require 'capistrano/rails/migrations'
+require 'capistrano/rails/assets'
+```
+
+When you run `cap production deploy`, it runs this following tasks:
+
+```
+deploy
+  deploy:starting
+    [before]
+      deploy:ensure_stage
+    deploy:check
+  deploy:started
+  deploy:updating
+    git:create_release
+    deploy:symlink:shared
+  deploy:updated
+    [before]
+      deploy:bundle
+    [after]
+      deploy:migrate
+      deploy:compile_assets
+      deploy:cleanup_assets
+      deploy:normalise_assets
+  deploy:publishing
+    deploy:symlink:release
+    deploy:restart
+  deploy:published
+  deploy:finishing
+    deploy:cleanup
+  deploy:finished
+    deploy:log_revision
+```
+
+For `cap production deploy:rollback`, it runs these tasks:
+
+```
+deploy
+  deploy:starting
+    [before]
+      deploy:ensure_stage
+    deploy:check
+  deploy:started
+  deploy:reverting
+    deploy:revert_release
+  deploy:reverted
+    [after]
+      deploy:rollback_assets
+  deploy:publishing
+    deploy:symlink:release
+    deploy:restart
+  deploy:published
+  deploy:finishing_rollback
+    deploy:cleanup_rollback
+  deploy:finished
+    deploy:log_revision
+```