Add a note to the README about instructional blog posts. (open-telemetry#1881)

Author: svrnm

README.md

@@ -28,6 +28,10 @@ You can submit a blog post either by forking this repository and writting it
 locally or by using the GitHub UI. In both cases we ask you to follow the
 instructions provided by the [blog post template](archetypes/blog.md).
 
+**Note**: Before writing a blog post, please ask yourself, if your content
+also might be a good addition to the documentation. If the answer is yes,
+create a new issue/PR with your content to get it added to the docs.
+
 ### Fork & Write locally
 
 Follow the [setup instructions][contributing.md] then, to create a skeletal blog