Include more files in PDM sdist builds

This was previously the job of directives in MANIFEST.in, which should
be covered by this PDM-specific configuration.

pyproject.toml

@@ -100,6 +100,14 @@ dev = [
     "tomli;python_version<'3.11'",
 ]
 
+[tool.pdm.build]
+source-includes = [
+    "CONTRIBUTING.rst",
+    "THANKS",
+    "docs/changelog.rst",
+    "samples/",
+]
+
 [build-system]
 requires = ["pdm-backend"]
 build-backend = "pdm.backend"