In Contest Builder under Notifications, you will have the option to send out contest launch notifications via email. Below is a screenshot of what this looks like in Contest Builder.
You'll need to select the box that says email. This will open up more options to customize the launch email. You will have the option to choose who gets the email (Participants and Observers/Participants/Observers). You will then need to fill out the message portion; there is a button with the option to use default, this will autofill this section. You can use the default text, add to the default text, or create a completely custom message to the users receiving this email.
Below is an example of what the email will look like when a contest is launched. If you choose to use recurring Leaderboard updates they will have the same format, but will include the leaderboard as it stands when the email is sent.