Re-organize contributor's guide (#4659)

* Reorganize contributor's doc

* Address comments from @trivialfis

* Address @sriramch's comment: include ABI compatibility guarantee

* Address @rongou's comment

* Postpone ABI compatibility guarantee for now

doc/contrib/docs.rst

@@ -0,0 +1,32 @@
+##########################
+Documentation and Examples
+##########################
+
+**Contents**
+
+.. contents::
+  :backlinks: none
+  :local:
+
+*********
+Documents
+*********
+* Documentation is built using `Sphinx <http://www.sphinx-doc.org/en/master/>`_.
+* Each document is written in `reStructuredText <http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html>`_.
+* You can build document locally to see the effect, by running
+
+.. code-block:: bash
+
+  make html
+
+inside the ``doc/`` directory.
+
+********
+Examples
+********
+* Use cases and examples will be in `demo <https://github.com/dmlc/xgboost/tree/master/demo>`_.
+* We are super excited to hear about your story, if you have blogposts,
+  tutorials code solutions using XGBoost, please tell us and we will add
+  a link in the example pages.
+
+