docs: v1 no longer maintained

README.md

@@ -18,7 +18,7 @@ See the below table on which version of React-Bootstrap you should be using in y
 | Bootstrap Version | React-Bootstrap Version | Documentation |
 | ------------- |:-------------:| -----:|
 | v5.x | 2.x | [Link][v5-documentation] |
-| v4.x | 1.x | [Link][v4-documentation] |
+| v4.x | 1.x (not maintained) | [Link][v4-documentation] |
 | v3.x | 0.33.x (not maintained) | [Link][v3-documentation] |
 
 ## Migrating from previous versions