TODO: document valid file paths

TODO.md

diff --git a/TODO.md b/TODO.md
index a7305aa..3b19ba3 100644
--- a/TODO.md
+++ b/TODO.md
@@ -80,6 +80,7 @@
 
 ## Documentation
 
+* document valid file paths
 * document: `zip_source_write()`: length can't be > `ZIP_INT64_MAX`
 * document: `ZIP_SOURCE_CLOSE` implementation can't return error
 * keep error codes in man pages in sync