Command mapping

Hey there, thanks for creating this gem 🙇

I've run into an issue when following the instructions in the README as the `shared_path` is not necessarily set correctly when you define the command mapping in the top-level scope.

Instead, what worked for me was to defer the mapping until after `deploy:starting`, where we can be sure the `shared_path` is initialized to the correct value:

```ruby
namespace :deploy do
# Add command mappings
after :starting, :map_commands do
SSHKit.config.command_map[:composer] = "php #{shared_path.join('composer.phar')}"
end

# Install composer (https://getcomposer.org/)
after :starting, 'composer:install_executable'

# …
end
```

Maybe it'd make sense to update the README and `composer:install_executable` task description?