Add a command for tracking parser updates

manual/src/upstream_parsers.md

@@ -66,4 +66,9 @@ example, the following command will update the Java parser:
 $ git subtree pull --prefix=vendor/tree-sitter-java git@github.com:tree-sitter/tree-sitter-java.git master
 ```
 
+To see when each parser was last updated, use the following shell
+command:
 
+```
+$ for d in $(git log | grep git-subtree-dir | tr -d ' ' | cut -d ":" -f2 | sort); do echo "$d"; git log --pretty="  %cs" -n 1 $d; done
+```