message: docs: add specsets to SidreDC schema
diff: @@ -39,6 +39,7 @@ The Axom project release numbers follow [Semantic Versioning](http://semver.org/
  - Inlet: Added support for deeply nested containers of structs
  - Inlet: Added support for `void` and strings in Lua-defined functions
  - Inlet: Added `get<std::vector<T>>` for retrieving arrays without index information
  +- Added support for registering material and species sets in `MFEMSidreDataCollection`
  ### Changed
  - The Sidre Datastore no longer rewires Conduit's error handlers to SLIC by default.
  