Document the LV class unification changes in the release notes

To give some basic hints to users as well as future developers and maintainers
of Blivet.

release_notes.rst

@@ -1,6 +1,49 @@
 2.0
 ====
 
+A single class for all LVs
+---------------------------
+
+In order to be better prepared for supporting things like *lvconvert*, Blivet
+now represents all LVs with a single class (keeping the name
+``LVMLogicalVolumeDevice``).
+
+
+Using the class
+++++++++++++++++
+
+In order to create LVs of various types, different values of the ``seg_type``
+parameter need to be passed. For example, to create a thin pool, ``thin-pool``
+segment type needs to be specified (optionally together with the
+thin-pool-specific parameters like ``metadata_size``) . The same applies to thin
+LVs and the ``thin`` segment type. To create a snapshot LV, one needs to specify
+the ``origin`` LV or set the ``vorigin`` flag to ``True``. Internal LVs require
+``parent_lv`` and ``int_type`` specifying the type of the internal LV.
+
+To determine the type of some LV, the newly added ``is_thin_lv``,
+``is_thin_pool``, ``is_snapshot_lv`` and ``is_internal_lv`` properties can be
+used.
+
+
+Implementation details
++++++++++++++++++++++++
+
+To avoid having a single gigantic class with hundreds of lines of code, the
+``LVMLogicalVolumeDevice`` class makes use of iheritance and "merges" together
+the ``LVMLogicalVolumeBase`` class and mixins for specific types of LVs (thin
+pool, thin LV,...) adding the type-specific methods and properties as well as
+type-specific implementations of various methods. The ``@type_specific``
+decorator makes sure that the right implementation of a method is called
+whenever there is a type-specific one (for example thin pools are created in a
+different way than good old linear LVs).
+
+The code that is common to all LVs lives in the ``LVMLogicalVolumeBase`` class
+together with properties that are required by this code. Type-specific code
+lives in the particular mixin classes and the generic/fallback implementations
+live in the (ultimate) ``LVMLogicalVolumeDevice`` class' methods decorated with
+the ``@type_specific`` decorator.
+
+
 LVM RAID
 ---------