Merge pull request #79 from ke-zhang-rd/doctr-doc-warning

DOC: update doctr instruction

Author: mrakitin

docs/source/publishing-docs.rst

@@ -15,6 +15,12 @@ We recommend `doctr <https://drdoctr.github.io/doctr/>`_ a tool that configures
 Travis-CI to automatically publish your documentation every time the master
 branch is updated.
 
+.. warning::
+    The repo you want to build the docs for has to be a root repo.
+    You cannot build docs for a forked repo by doctr yet.
+    The doctr team is working on enabling a forked repo build under
+    `PR #343 <https://github.com/drdoctr/doctr/pull/343>`_.
+
 Install doctr.
 
    .. code-block:: bash