README: clarify that Medium "members only" articles are undesirable

Author: ericseppanen

README.md

@@ -39,7 +39,7 @@ What we are generally NOT looking for includes:
 * Rants or anything degrading to any part or member of the Community. Rather than submitting an article about what is wrong with something, we would much rather you write something that explains how you'd make it better.
 * Duplicates of recent posts (even with the wording changed slightly)
 * Links to crates or GitHub repos without some sort of context. We would much rather you submit a blog post introducing your project and how a Rust user might use it (and what it would help them do) and/or what you learned about Rust in the process of writing the project.
-* Anything behind a paywall (this includes Medium's paid article mechanism)
+* Anything behind a paywall (this includes Medium's paid article / members-only mechanism)
 * Anything that requires information to be shared/captured (like an email address) in order to access
 
 These are meant to be guidelines, if you are ever not sure about whether something should be included please feel free to open a pull request anyway and we can discuss it!