Add some documentation for stock control feature. Fixes #169

docs/advanced-usage.rst

@@ -173,3 +173,17 @@ Example - don't allow batteries to be stacked
                 return true;
             }
         }
+
+Limited supply boxes
+--------------------
+
+In standard/basic use, BoxPacker will assume you have an adequate enough supply of each box type on hand to cover all
+eventualities i.e. your warehouse will be very well stocked and the concept of "running low" is not applicable.
+
+However, if you only have limited quantities of boxes available and you have accurate stock control information, you can
+feed this information into BoxPacker which will then take it into account so that it won't suggest a packing which would
+take you into negative stock.
+
+To do this, have your box objects implement the ``BoxPacker\LimitedSupplyBox`` interface which has a single additional method
+over the standard ``BoxPacker\Box`` namely ``getQuantityAvailable()``. The library will automatically detect this and
+use the information accordingly.