doc about other pep517 backend

docs/docs/usage/advanced.md

@@ -135,3 +135,15 @@ Testing:
     For GitHub Action users, there is a [known compatibility issue](https://github.com/actions/ virtual-environments/issues/2803) on Ubuntu virtual environment.
     If PDM parallel install is failed on that machine you should either set `parallel_install` to `false` or set env `LD_PRELOAD=/lib/x86_64-linux-gnu/libgcc_s.so.1`.
     It is already handled by the `pdm-project/setup-pdm` action.
+
+## Use other PEP 517 backends
+
+PDM supports ALL PEP 517 build backends that comply with PEP 621 specification. At the time of writing, `flit` is going to switch to PEP 621 metadata in the near future, then you can keep `flit` as the build-backend while still managing dependencies using PDM:
+
+```toml
+[build-system]
+requires = ["flit_core >=2,<4"]
+build-backend = "flit_core.buildapi"
+```
+
+PDM will call the correct backend when `pdm build`.