Document g:ale_typescript_tslint_rules_dir

doc/ale-typescript.txt

@@ -28,6 +28,14 @@ g:ale_typescript_tslint_config_path       *g:ale_typescript_tslint_config_path*
 
   ALE will first discover the tslint.json path in an ancestor directory. If no
   such path exists, this variable will be used instead.
+  
+  
+g:ale_typescript_tslint_rules_dir         *g:ale_typescript_tslint_rules_dir*
+                                          *b:ale_typescript_tslint_rules_dir*
+  Type: |String|
+  Default: `''`
+
+  If this variable is set, ALE will use it as the rules directory for tslint.
 
 
 g:ale_typescript_tslint_use_global         *g:ale_typescript_tslint_use_global*