Will it add the text at the bottom of an email thats done in Yahoo Mail on the web?
No, avast can't edit web pages that are displayed in your browser.
Yahoo is not a pop3 email service, it is web based (so the Internet Mail provider doesn't directly protect it). Web based email is simply your email being viewed in the same way you browser the Internet. The pages (that display your email) are downloaded into your Temporary Internet folder, just like regular web pages and displayed on your browser screen.
Yahoo does I believe scan emails that come into it's inbox, but you would have to confirm that yourself.
Set-up the standard shield to High, that way all files created/modified will be scanned.