Clarify the blog post template headers

Author: apiraino

README.md

@@ -40,3 +40,13 @@ Code of Conduct, see `CODE_OF_CONDUCT.md` for more.
 Please send pull requests to the master branch. If you're trying to do
 something big, please open an issue before working on it, so we can make sure
 that it's something that will eventually be accepted.
+
+When writing a new blog post, keep in mind the file headers:
+```
+---
+layout: post
+title: Title of the blog post
+author: Blog post author (or on behalf of which team)
+release: true (to be only used for official posts about Rust releases announcements)
+---
+```