Add a simple comment about some string wizardry

src/services/post-file.ts

@@ -13,6 +13,8 @@ export async function readPostFile(path: string): Promise<Post> {
   let slug = parsedData.slug;
 
   if (!slug) {
+    // get the last object in the path, remove the last three characters,
+    // presuming they are `.md`
     slug = path.split("/").pop()?.slice(0, -3);
   }