Improve deployment instructions

Capistrano won't symlink the `public/system` folder by default anymore.
Capistrano needs to be configured accordingly.

See related issues:

* https://github.com/capistrano/capistrano/commit/033bf8ee492936787cd0744e69b3df8a5ad5b6e6#diff-348edd3e734eaeed0fe97af037f3c647
* https://github.com/capistrano/rails/issues/104
* https://github.com/capistrano/rails#symlinks

[fixes #2141]
[fixes #2166]

README.md

@@ -53,6 +53,7 @@ https://github.com/thoughtbot/paperclip/releases
   - [Dynamic Processors:](#dynamic-processors)
   - [Dynamic Processors:](#dynamic-processors)
 - [Logging](#logging)
 - [Logging](#logging)
 - [Deployment](#deployment)
 - [Deployment](#deployment)
+  - [Attachment Styles](#attachment-styles)
 - [Testing](#testing)
 - [Testing](#testing)
 - [Contributing](#contributing)
 - [Contributing](#contributing)
 - [License](#license)
 - [License](#license)
@@ -566,8 +567,8 @@ The files that are assigned as attachments are, by default, placed in the
 directory specified by the `:path` option to `has_attached_file`. By default, this
 directory specified by the `:path` option to `has_attached_file`. By default, this
 location is `:rails_root/public/system/:class/:attachment/:id_partition/:style/:filename`.
 location is `:rails_root/public/system/:class/:attachment/:id_partition/:style/:filename`.
 This location was chosen because, on standard Capistrano deployments, the
 This location was chosen because, on standard Capistrano deployments, the
-`public/system` directory is symlinked to the app's shared directory, meaning it
-will survive between deployments. For example, using that `:path`, you may have a
+`public/system` directory can be symlinked to the app's shared directory, meaning it
+survives between deployments. For example, using that `:path`, you may have a
 file at
 file at
 
 
     /data/myapp/releases/20081229172410/public/system/users/avatar/000/000/013/small/my_pic.png
     /data/myapp/releases/20081229172410/public/system/users/avatar/000/000/013/small/my_pic.png
@@ -829,6 +830,16 @@ More information in the [rdocs](http://www.rubydoc.info/github/thoughtbot/paperc
 Deployment
 Deployment
 ----------
 ----------
 
 
+To make Capistrano symlink the `public/system` directory so that attachments
+survive new deployments, set the `linked_dirs` option in your `config/deploy.rb`
+file:
+
+```ruby
+set :linked_dirs, fetch(:linked_dirs, []).push('public/system')
+```
+
+### Attachment Styles
+
 Paperclip is aware of new attachment styles you have added in previous deploys. The only thing you should do after each deployment is to call
 Paperclip is aware of new attachment styles you have added in previous deploys. The only thing you should do after each deployment is to call
 `rake paperclip:refresh:missing_styles`.  It will store current attachment styles in `RAILS_ROOT/public/system/paperclip_attachments.yml`
 `rake paperclip:refresh:missing_styles`.  It will store current attachment styles in `RAILS_ROOT/public/system/paperclip_attachments.yml`
 by default. You can change it by:
 by default. You can change it by: