add docs for quickfort burrow blueprint mode

docs/guides/quickfort-user-guide.rst

@@ -108,6 +108,10 @@ Feature summary
     - Set building properties (such as a name)
     - Can attach and configure track stops as part of hauling routes
 
+- Burrow mode
+
+    - Supports creating, adding to, and subtracting from burrows.
+
 Introduction to blueprints
 --------------------------
 
@@ -866,6 +870,35 @@ names an existing route, the stop will be added to that route::
 These two track stops (which do not dump their contents) simply exist on a
 common route at the ends of a connected carved track.
 
+#burrow mode
+------------
+
+``#burrow`` mode can create, extend, and remove tiles from burrows.
+
+Burrow designation syntax
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The syntax should look familiar by now::
+
+    symbol{properties}(expansion)
+
+See the `#burrow mode reference`_ for symbol and property definitions.
+
+Here's how to create (or add to, if a burrow by that name already exists) a
+5x5 burrow named ``Inside+``::
+
+    #burrow
+    a{create=true name=Inside+}(5x5)
+
+Why the trailing ``+``? That's to indicate to the `burrow` plugin that the
+burrow should grow as adjacent tiles are dug out.
+
+Similarly, here is how to erase a tile from all burrows that currently include
+it::
+
+    #burrow
+    e
+
 .. _quickfort-modeline:
 
 Modeline markers
@@ -2199,3 +2232,17 @@ Symbol            Type                          Properties
 ``trackrampSEW``  track ramp tee to the S, E, W
 ``trackrampNSEW`` track ramp cross
 ================= ============================= ==========
+
+#burrow mode reference
+~~~~~~~~~~~~~~~~~~~~~~
+
+====== ======= ==========
+Symbol Meaning Properties
+====== ======= ==========
+``a``  add     ``name``: if set, will add to an existing burrow of this name.
+               ``create``: if set to ``true``, will create a burrow with the
+               specified ``name`` if it doesn't already exist.
+``e``  erase   ``name``: if set, will only affect the first burrow of the given
+               name. if not set, will affect all burrows that cover the given
+               tiles.
+====== ======= ==========