docs: add documentation about events

Signed-off-by: Philip Molares <philip.molares@udo.edu>
Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

docs/content/dev/events.md

@@ -0,0 +1,9 @@
+In HedgeDoc we use an event system based on [EventEmitter2][eventemitter2]. It's used to reduce circular dependencies between different services and inform these services about changes.
+
+HedgeDoc's system is basically [the system NestJS offers][nestjs/eventemitter].  
+The config for the `EventEmitterModule` is stored in `events.ts` and exported as `eventModuleConfig`.  
+In the same file enums for the event keys are defined. Each of these events is expected to be sent with an additional value. In the enum defintion a comment should tell you what exactly this value should be.
+
+[eventemitter2]: https://github.com/EventEmitter2/EventEmitter2
+[nestjs/eventemitter]: https://docs.nestjs.com/techniques/events
+

src/events.ts

@@ -15,6 +15,6 @@ export const eventModuleConfig = {
 };
 
 export enum NoteEvent {
-  PERMISSION_CHANGE = 'note.permission_change',
+  PERMISSION_CHANGE = 'note.permission_change' /** noteId: The id of the [@link Note], which permissions are changed. **/,
-  DELETION = 'note.deletion',
+  DELETION = 'note.deletion' /** noteId: The id of the [@link Note], which is being deleted. **/,
 }