Add virtualenv option for contributors in README. (#927)

Signed-off-by: Michael Vincerra <michael.vincerra@intel.com>

Revise Makefile to enable make venv target and add instructions.

Signed-off-by: Michael Vincerra <michael.vincerra@intel.com>

Revises README.md and root-level Makefile target `make venv`.

Signed-off-by: Michael Vincerra <michael.vincerra@intel.com>

Revise venv target; revise README to simplify activation/deactivation.

Signed-off-by: Michael Vincerra <michael.vincerra@intel.com>

Revise syntax to emph markdown rendering.

Signed-off-by: Michael Vincerra <michael.vincerra@intel.com>

Fix formatting errors and make consistent ref to platforms.

Signed-off-by: Michael Vincerra <michael.vincerra@intel.com>

Makefile

@@ -34,7 +34,6 @@ clean:
 	rm -rf venv
 
 venv:
-	virtualenv -p python$(PY_VERSION) venv;
+	virtualenv venv;\
 	source venv/bin/activate; \
 	pip3 install -r requirements.txt;
-

README.rst

@@ -67,6 +67,87 @@ If you want to build the documentation exactly as seen on the website, use
 ``make py`` followed by ``make htmlall``. This builds some
 external dependencies and all supported languages.
 
+Use virtualenv 
+**************
+
+To develop documentation in a ``virtualenv``, use the ``venv`` target.
+The Clear Linux\* OS documentation make target ``venv`` provides a 
+simple development environment that ensures that you have the 
+latest packages and that you manage Python versions separately. Use of the 
+``virtualenv`` requires **Python 3.6** or higher. For Windows examples below, use Powershell as an Administrator.
+
+The version of Python used to **create the virtual environment** is the 
+default python version **used in the virtual environment**. 
+
+#. Verify ``pip`` is installed. A file path to pip should appear. 
+
+   * On Clear Linux OS and macOS\*:
+
+   .. code-block:: bash
+
+      which pip
+   
+   On Windows\* 10 OS: 
+
+   .. code-block:: bash
+
+      pip --version
+
+#. If ``pip`` is not installed, install it. 
+
+   * On Clear Linux OS and macOS:
+
+   .. code-block:: bash
+
+      python3 -m pip install --user --upgrade pip
+
+   * On Windows 10 OS: 
+
+   .. code-block:: bash
+
+      py -m pip install --upgrade pip
+
+   .. note::
+
+      This assumes Python was already added to your Windows path. 
+
+Install virtualenv 
+------------------
+
+#. Install ``virtualenv``. 
+
+   * On Clear Linux\* OS and macOS\*:
+
+     .. code-block:: bash
+
+        python3 -m pip install --user virtualenv
+
+   * On Windows 10 OS: 
+
+   .. code-block:: bash
+
+      py -m pip install --user virtualenv
+
+#. Create the ``virtualenv`` and install the required packages: 
+
+   .. code-block:: bash
+
+      make venv
+
+ #. Activate the ``venv``. 
+
+    .. code-block:: bash
+
+       source venv/bin/activate
+
+   Follow `Run the build`_ section to start developing documentation.
+
+#. Remove the ``venv`` when finished developing.  
+
+   .. code-block:: bash
+
+      deactivate
+
 Additional help
 ***************
 
@@ -80,7 +161,8 @@ build before building again by running ``make clean``:
 
    make clean
 
-This will completely remove the previous build output.
+This will completely remove the previous build output, including artifacts 
+from the `make venv` target when done outside an active venv.
 
 Convenience script
 ==================