I would like to promote BeWelcome by adding a link to the signature of my email. How do I do that?

How to add a signature to your emails will vary depending on your email provider. Usually, there is a link for mail Options or Preferences where a signature can be created. Also, once the signature is created, some email providers require you to add the signature to the end of each email yourself, while others will add it automatically. You will have to check out how your particular email provider does this.
* If you use MS Outlook, go to "tools" > "options" > signature and then add a signature, and select for which emails it will be automatically included, or if you want to add it manually in each email, hitting the "signature" button
* If you use Thunderbird, first you have to create a .txt file (with notepad e.g.) that contains the text of your signature. Then go to "tools" > "account settings", and for the account you choose tick the "join a signature" box, and select the address of the txt file you created.