pyproject.toml: Add comment

pyproject.toml

+# This file defines options needed for the operation of our code formatting
+# tools, Black and isort.
+
+[tool.black]
+target_version = ['py38']
+
 [tool.isort]
 # Be compatible with black, the other linter
 profile = "black"
@@ -9,7 +15,5 @@ atomic = true
 # Don't complain about gitignored files
 skip_gitignore = true
 
+# Ignore these directories
 skip_glob = ["result*", "venv"]
-
-[tool.black]
-target_version = ['py38']