You can use a normal Formula field to do this:
HYPERLINK('mailto:'+EmailAddress__c+'?subject=Hello+There&body=This+is+an+example','Email Me')
For more information, see Email Links on MDN.
You can also make a quick action:
<aura:component implements="force:lightningQuickAction,force:hasRecordId" >
<aura:attribute name="contact" type="Contact" />
<force:recordData recordId="{!v.recordId}" targetFields="{!v.contact}" layoutType="FULL" recordUpdated="{!c.sendEmail}" />
</aura:component>
({
sendEmail: function(component, event, helper) {
var a = document.createElement("a"),
contact = component.get("v.contact");
a.href = "mailto:"+encodeURIComponent(contact.Email)+
"?subject=Test%20subject"+
"&body="+encodeURIComponent(contact.Description);
a.click();
$A.get("e.force:closeQuickAction").fire();
}
})