Add link for SPDX license expression

Author: cdce8p

Diff for: docs/config/metadata.md

@@ -99,7 +99,8 @@ requires-python = ">=3.8"
 
 ## License
 
-For more information, see [PEP 639][]. `license` must be a valid SPDX license expression,
+For more information, see [PEP 639][]. `license` must be a valid
+[SPDX license expression](https://peps.python.org/pep-0639/#spdx-license-expression-syntax),
 `license-files` a list of glob patterns which match license files. If not specified,
 `license-files` will default to `['LICEN[CS]E*', 'COPYING*', 'NOTICE*', 'AUTHORS*']`.