The idea behind this feature is that sometimes you want to give your users the ability to email documents from SharePoint and sometimes you don't.
This feature allows you to turn on this button per library, so it is not available on every library. It also gives you the ability to limit the total size of files and also what level of permission the user needs to have in order to send the email.
1. Deploy the .wsp file as normal.
2. Publish the solution in central administration.
3. Go to the site you want to allow users to send emails from and activate the site feature.
4. Go to the list or library you want users to be able to send documents from and go to the list or library settings. Under the communication section you will have a new option for email attachment settings.
5. On the email attachments settings page click the enable radio button and click save.
Download the detailed instructions here:
Now when you go back to the library you will have a new button added to the ribbon. There is also an option added to the context menu of the list or library items.