docs: Add README.md to zulip/docs to prevent reading on GitHub.

This should help avoid confusing about broken links.

Tweaked by tabbott to fix formatting and the linter.

docs/README.md

@@ -0,0 +1,6 @@
+# Zulip markdown documentation hosted elsewhere
+
+The markdown files in this directory ( /zulip/docs ) are not intended
+to be read on GitHub. Instead, visit our
+[ReadTheDocs](https://zulip.readthedocs.io/en/latest/index.html) to
+read the Zulip documentation.