Add documentation of LMS/Studio features usable by XBlocks

[LMS-192]

docs/developers/source/xblocks.rst

+Integrating XBlocks with edx-platform
+=====================================
+
+The edX LMS and Studio have several features that are extensions of the core XBlock
+libraries (https://xblock.readthedocs.org). These features are listed below.
+
+LMS
+---
+
+Runtime Features
+~~~~~~~~~~~~~~~~
+
+These are properties and methods available on ``self.runtime`` when a view or handler is executed by the LMS.
+
+* anonymous_student_id: An identifier unique to the student in the particular course
+  that the block is being executed in. The same student in two different courses
+  will have two different ids.
+
+* publish(event): Emit events to the surrounding system. Events are dictionaries with
+  at least the key 'event_type', which identifies the other fields.
+
+  TODO: Link to the authoritive list of event types.
+
+In the future, these are likely to become more formal XBlock services (one related to users,
+and the other to event publishing).
+
+Class Features
+~~~~~~~~~~~~~~
+
+These are class attributes or functions that can be provided by an XBlock to customize behaviour
+in the LMS.
+
+* student_view (XBlock view): This is the view that will be rendered to display
+  the XBlock in the LMS.
+* has_score (class property): True if this block should appear in the LMS progress page.
+* get_progress (method): See documentation in x_module.py:XModuleMixin.get_progress.
+* icon_class (class property): This can be one of (``other``, ``video``, or ``problem``), and
+  determines which icon appears in edx sequence headers. There is currently no way to provide
+  a different icon.
+
+Grading
+~~~~~~~
+
+To participate in the course grade, an XBlock should set ``has_score`` to ``True``, and
+should ``publish`` a ``grade`` event whenever the grade changes. The ``grade`` event is a
+dictionary of the following form::
+
+    {
+        'event_type': 'grade',
+        'value': <number>,
+        'max_value': <number>,
+    }
+
+The grade event represents a grade of ``value/max_value`` for the current user.
+
+Restrictions
+~~~~~~~~~~~~
+
+* A block can't modify the value of any field with a scope where the ``user`` property
+  is ``UserScope.NONE``.
+
+Studio
+------
+
+Class Features
+~~~~~~~~~~~~~~
+
+* studio_view (XBlock.view): The view used to render an editor in Studio.
+
+* non_editable_metadata_fields (property): A list of xblock.fields.Field objects that
+  shouldn't be displayed in the default editing view for Studio.
+
+Restrictions
+~~~~~~~~~~~~
+
+* A block can't modify the value of any field with a scope where the ``user`` property
+  is not ``UserScope.NONE``.
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