Add note for setuptools_scam fallback version

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

pyproject.toml

@@ -3,6 +3,8 @@ requires = ["setuptools >= 50", "wheel", "setuptools_scm[toml] >= 4"]
 build-backend = "setuptools.build_meta"
 
 [tool.setuptools_scm]
+# this is used populated when creating a git archive
+# and when there is .git dir and/or there is no git installed
 fallback_version = "v9999.$Format:%h-%cs$"
 
 [tool.pytest.ini_options]