Put Twitter guidelines into writing

These were previously already published as tweets:
https://twitter.com/rustlang/status/1263105812064620545

Author: badboy

src/SUMMARY.md

@@ -10,6 +10,7 @@
       - [Triagebot](./platforms/zulip/triagebot.md)
 - [Core](./core/README.md)
     - [Rust Blog Guidelines](./core/blogs.md)
+    - [Twitter Guidelines](./core/twitter.md)
 - [Community](./community/README.md)
     - [State of Rust Survey FAQ](./community/survey-faq.md)
 - [Compiler](./compiler/README.md)

src/core/twitter.md

@@ -0,0 +1,22 @@
+# Twitter Guidelines
+
+The project runs the Twitter account [@rustlang](https://twitter.com/rustlang).
+The account is handled by a small team of volunteers.
+
+The account will mostly tweet links to the Rust blog and Rust Insiders blog.
+Additionally it will retweet:
+
+* links to blog posts about Rust, retweeting the original author if possible,
+* questions about Rust, so all followers can help,
+* Meetup or conference announcements,
+* anything else relevant.
+
+The Direct Messages are open to everyone.
+If someone wants something retweeted, they should send the tweet via DM.
+The vast majority of these things should be retweeted, keeping it to the above rules.
+Requests of an author via DM or Tweet to not retweet something will be honored.
+
+Additionally account handlers may look through the [#rustlang](https://twitter.com/hashtag/rustlang?src=hashtag_click) hashtag for noteworthy content.
+
+The account will only follow a small number of Project-owned/related Twitter accounts.
+At the time of writing (February 2022) this is only [@cratesiostatus](https://twitter.com/cratesiostatus) and [@rust_foundation](https://twitter.com/rust_foundation).