Slots are part of the native Web Component. Because LEGO builds native Web Components, you can use the standard slots as documented.
Example #
index.html
<user-profile>
<span>This user is in Paris</span>
</user-profile>
bricks/user-profile.html
<template>
<h1>User profile</h1>
<p>Important information: <slot></slot></p>
</template>
Outputs:
<h1>User profile</h1>
<p>Important information: <span>This user is in Paris</span></p>