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.

tools/linter_lib/custom_check.py

@@ -743,9 +743,9 @@ def build_custom_checkers(by_lang):
         {'pattern': '\[(?P<url>[^\]]+)\]\((?P=url)\)',
          'description': 'Linkified markdown URLs should use cleaner <http://example.com> syntax.'},
         {'pattern': 'https://zulip.readthedocs.io/en/latest/[a-zA-Z0-9]',
-         'exclude': ['docs/overview/contributing.md', 'docs/overview/readme.md'],
+         'exclude': ['docs/overview/contributing.md', 'docs/overview/readme.md', 'docs/README.md'],
          'include_only': set(['docs/']),
-         'description': "Use relatve links (../foo/bar.html) to other documents in docs/",
+         'description': "Use relative links (../foo/bar.html) to other documents in docs/",
          },
         {'pattern': '\][(][^#h]',
          'include_only': set(['README.md', 'CONTRIBUTING.md']),