Update blog.md with clearer instructions (open-telemetry#3548)

Co-authored-by: Severin Neumann <severin.neumann@altmuehlnet.de>
Co-authored-by: svrnm <neumanns@cisco.com>
Co-authored-by: Phillip Carter <pcarter@fastmail.com>

Author: 3 people

archetypes/blog.md

@@ -17,7 +17,8 @@ With contributions from secondary-author-name-1, ..., and secondary-author-n.
 
 ## Top-level heading
 
-Top-level headings start at **level 2**, as shown above.
+Top-level headings start at **level 2**. This means, that your post should not
+include `# headings` for top-level headings but `## headings` instead.
 
 ## Paragraphs
 
@@ -44,3 +45,6 @@ npm run format
 ```
 
 Happy writing!
+
+**Note:** If you view this page with the GitHub file viewer, you can safely
+ignore the `Error in user YAML` at the top of this page.