add more information about the GNU compat (#3410)

* add more information about the GNU compat

Co-authored-by: Terts Diepraam <[email protected]>

README.md

@@ -398,7 +398,15 @@ To contribute to uutils, please see [CONTRIBUTING](CONTRIBUTING.md).
 
 ## Utilities
 
-| Done | Semi-Done | To Do |
+Please note that this is not fully accurate:
+* Some new options can be added / removed in the GNU implementation;
+* Some error management might be missing;
+* Some behaviors might be different.
+
+See https:/uutils/coreutils/issues/3336 for the main meta bugs
+(many are missing).
+
+| Done | WIP | To Do |
 |-----------|-----------|--------|
 | arch | cp | stty |
 | base32 | date | |
@@ -417,8 +425,8 @@ To contribute to uutils, please see [CONTRIBUTING](CONTRIBUTING.md).
 | cut | tac | |
 | dircolors | tail | |
 | dirname | test | |
-| du |   | |
-| echo |   | |
+| du | dir | |
+| echo | vdir | |
 | env | | |
 | expand | | |
 | factor | | |