chore(issues): add event type proposal template

.github/ISSUE_TEMPLATE/event-type-proposal.md

@@ -0,0 +1,20 @@
+---
+name: Event type proposal
+about: Suggest a type of event you'd like to see implemented in Kosmorro
+title: ''
+labels: "enhancement, \U0001F320 events"
+assignees: ''
+
+---
+
+**Event type**: 
+**Involved objects (check the boxes)**:
+- [ ] Sun
+- [ ] Earth's moon
+- [ ] Planets
+
+**Definition of the event:**
+
+<!-- Replace this comment with the official definition of the event type, with a link to its source. -->
+
+**Other useful information** (implementation proposal, etc):