Document _ZO_EXCLUDE_DIRS env var

README.md

@@ -101,6 +101,9 @@ zoxide init fish | source
 
 ### Environment variables
 
-- `$_ZO_ECHO`: `z` will print the matched directory before navigating to it
 - `$_ZO_DATA`: sets the location of the database (default: `~/.zo`)
+- `$_ZO_ECHO`: `z` will print the matched directory before navigating to it
+- `$_ZO_EXCLUDE_DIRS`: list of directories separated by platform-specific
+    characters (`:` on Linux and macOS, and `;` on Windows) to be excluded from
+    the database
 - `$_ZO_MAXAGE`: sets the maximum total rank after which entries start getting deleted