Implement write buffering (#22)

docs/udps/intro.rst

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+Introduction
+============
+
+Although the official SystemRDL spec defines numerous properties that allow you
+to define complex register map structures, sometimes they are not enough to
+accurately describe a necessary feature. Fortunately the SystemRDL spec allows
+the language to be extended using "User Defined Properties" (UDPs). The
+PeakRDL-regblock tool understands several UDPs that are described in this
+section.
+
+
+.. list-table:: Summary of UDPs
+    :header-rows: 1
+
+    *   - Name
+        - Component
+        - Type
+        - Description
+
+    *   - buffer_writes
+        - reg
+        - boolean
+        - If set, writes to the register are double-buffered.
+
+          See details here: :ref:`write_buffering`.
+
+    *   - wbuffer_trigger
+        - reg
+        - reference
+        - Defines the buffered write commit trigger.
+
+          See details here: :ref:`write_buffering`.