add doc for UserConfigDirOrDefault

folder/README.md

@@ -1,2 +1,10 @@
 # folderutil
-The package contains various helpers to interact with folders
+The package contains various helpers to interact with folders
+
+## UserConfigDirOrDefault
+
+UserConfigDirOrDefault returns the default root directory to use for user-specific configuration data. Users should create their own application-specific subdirectory within this one and use that.
+
+On Unix systems, it returns $XDG_CONFIG_HOME as specified by https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if non-empty, else $HOME/.config. On Darwin, it returns $HOME/Library/Application Support. On Windows, it returns %AppData%. On Plan 9, it returns $home/lib.
+
+If the location cannot be determined (for example, $HOME is not defined), then it will return given value as default.