add nestsets documentation to blueprint docs (#338)

src/docs/sphinx/blueprint_mesh.rst

@@ -508,6 +508,35 @@ To put it in short, each entry in the ``specsets`` section of the Blueprint hier
 
 
 
+Nesting Sets
+++++++++++++++++++++
+
+Nesting Sets are used to represent the nesting relationships between different domains in multi-domain mesh environments. Most commonly, this subset of the Blueprint specification is used for AMR (adaptive mesh refinement) meshes.
+
+Each entry in the Nesting Sets section contains an independent set of nesting relationships between domains in the described mesh.
+On an individual basis, a nesting set contains a source topology, an element association, and a list of nesting windows.
+The windows for a particular nesting set describe the topological nesting pattern for a paired set of domains, which includes the ID of the partnered domain, the type of the partnered domain (parent or child), and the self-relative origin and dimensions of the nesting relationship.
+The Blueprint schema for each entry in the ``nestsets`` section matches the following template:
+
+   * nestsets/nestset/association: "vertex" | "element"
+   * nestsets/nestset/topology: "topo"
+   * nestsets/nestset/windows/window/domain_id: (integer)
+   * nestsets/nestset/windows/window/domain_type: "parent" | "child"
+   * nestsets/nestset/windows/window/ratio/{i, j, k}
+   * nestsets/nestset/windows/window/origin/{i, j, k}
+   * nestsets/nestset/windows/window/dims/{i, j, k}
+
+Each domain that contains a Nesting Sets section must also update its State section to include the domain's global nesting level.
+This additional requirement adds the follow constraint to the ``state`` section:
+
+   * state/level_id: (integer)
+
+.. note::
+   The Nesting Sets section currently only supports nesting specifications for
+   structured topologies. There are plans to extend this feature to support
+   unstructured topologies in future versions of Conduit.
+
+
 Adjacency Sets
 ++++++++++++++++++++