docs: add a readme on how tweets folder is meant to be used

tweets/README.md

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+# The tweets/ folder
+
+To create a new tweet create a new `*.tweet` file in this `tweets/` folder.
+
+<kbd>[Create new tweet](../../../new/main/?filename=tweets/<your-path>.tweet)</kbd>
+
+## Example
+
+Create a new file `tweets/hello-world.tweet` with the content
+
+> Hello, world!
+
+You can use subfolders, e.g. `tweets/2019-02/hello-world.tweet`, as long as the file is in the `tweets/` folder and has the `.tweet` file extension
+
+## Notes
+
+- Only newly created files are handled, deletions, updates or renames are ignored.
+- `*.tweet` files will not be created for tweets you send out directly from twitter.com
+- If you need to rename an existing tweet file, please do so locally using [`git mv old_filename new_filename`](https://help.github.com/en/articles/renaming-a-file-using-the-command-line), otherwise it may occur as deleted and added which would trigger a new tweet.
+
+## Questions?
+
+The full docs on twitter together are here: https://github.com/marketplace/actions/twitter-together