Started docs for feature detection

docs/image_feature_detection.rst

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+=======================
+Image Feature Detection
+=======================
+
+Wagtail has the ability to automatically detect faces and features inside your images and automatically crop the images to those features.
+
+Feature detection is used by the ``fill`` image filter. An image that has had feature detection run on it would be cropped based on the features that were detected.
+
+
+Setup
+=====
+
+Feature detection requires OpenCV which can be a bit tricky to set up as it is not currently pip-installable.
+
+
+Installing OpenCV on Debian/Ubuntu
+----------------------------------
+
+Debian and ubuntu provide python-opencv as a package installable with apt-get:
+
+ .. code-block:: bash
+
+    sudo apt-get install python-opencv python-numpy
+
+
+If you are using a Virtual environment, you will need to make sure that you have site packages enabled.
+
+For Python 3, go into your virtualenv directory and open the ``pyvenv.cfg`` file then set ``include-system-site-packages`` to ``true``.
+
+If you are using Virtualenv, go into your virtualenv directory and delete a file called ``lib/python-x.x/no-global-site-packages.txt``.
+
+
+You can test that its all working by opening up a python shell (with your virtual environment active) and typing:
+
+ .. code-block:: python
+
+    import cv
+
+
+Switching on feature detection
+------------------------------
+
+To switch on feature detection, set the ``WAGTAIL_FEATURE_DETECTION_ENABLED`` setting to ``True``:
+
+ .. code-block:: python
+
+    # settings.py
+
+    WAGTAIL_FEATURE_DETECTION_ENABLED = True
+
+
+Feature detection runs when images are uploaded so any images you already have will behave as they used to.
+
+If you have images in your site that you want to have