Add post on python 3.6 support

This will be used to link python 3.6 beta test email list

Author: agjohnson

build-image-upgrade.rst

@@ -21,7 +21,12 @@ versions are instead installed from source, using `pyenv`_.
 
 If you'd like beta access to this build image for your projects, you can sign
 up here:
-https://goo.gl/forms/AKEoeWHixlzVfqKT2
+
+.. admonition:: Update
+
+    Users are now able to opt in to a build image that support Python 3.6,
+    signing up for beta access is no longer required. For more information, see
+    our blog post on :doc:`/python-36-support`
 
 We are slowly moving project configuration to our `YAML configuration file`_,
 ``readthedocs.yml``. Access to specific versions of Python will only be

conf.py

@@ -226,6 +226,12 @@
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'ReadtheDocsBlogdoc'
 
+intersphinx_mapping = {
+    'readthedocs': (
+        'https://docs.readthedocs.io/en/latest',
+        None
+    ),
+}
 
 # -- Options for LaTeX output ---------------------------------------------
 

python-36-support.rst

@@ -0,0 +1,43 @@
+.. post:: Mar 29, 2018
+    :author: Anthony
+
+Python 3.6 Support
+==================
+
+A long time back, :doc:`we wrote about </build-image-upgrade>` started testing
+a new build image that uses `pyenv`_ to support multiple versions of Python.
+Until recently, we were selectively opting projects in to help test the new
+image, but at the beginning of the year, we added a configuration option to
+allow projects to opt in to using the new image before we make it the default
+build image.
+
+In the near future, this build image will be the default build image, but for
+now, you can manually opt your project in using our
+:doc:`YAML configuration file <readthedocs:yaml-config>`.
+
+What you need
+-------------
+
+In order to use Python 3.6, you need to select our ``latest`` build image, and
+you need to select to use Python 3.6. The build image is controlled with the
+:ref:`readthedocs:yaml__build__image` setting in the YAML config, and you can
+set the Python version with the :ref:`readthedocs:yaml__python__version`
+setting.
+
+Your project's ``readthedocs.yml`` file should contain::
+
+    build:
+        image: latest
+
+    python:
+        version: 3.6
+
+If you run in to any problems building, you might also need to wipe your version
+for now. We plan to have some of the issues switching Python versions and build
+images resolved before making a move to a new build image.
+
+As always, if you notice anything strange with your build, feel free to raise an
+issue on `our issue tracker`_.
+
+.. _pyenv: https://github.com/yyuu/pyenv
+.. _our issue tracker: https://github.com/rtfd/readthedocs.org/issues