To do this you just need to add the following code to the HTML section of your chosen Web Page.
<a href="mailto:firstname.lastname@example.org?subject=Email%20Subject%20Here&body=Body Message Here?">Email Me</a>
..or if you'd like your actual email address to display in the body of the page rather than the "Email Me" text, then use this..
<a href="mailto:email@example.com?subject=Email%20Subject%20Here&body=Body Message Here">firstname.lastname@example.org</a>
(Note that you can edit the code above to suit your own choice of email "Subject", and "Body" text.)
Simply follow these steps to edit the HTML of any of your Web Pages (Home, About, Contact etc..)
Now select the Web Page you'd like to edit Eg: Home, About, etc..
Now hit the Edit HTML Source button to the top right of the text editor.
Make your changes and then hit Update to the bottom left of the HTML source editor window.
Finally hit the green Save button to the bottom right of the Edit Page to confirm all your changes.
..and you're Done!