examples/docs: caveat that PL examples don't work on TPU (#8309)

examples/README.md

@@ -63,6 +63,7 @@ very detailed [pytorch/xla README](https://github.com/pytorch/xla/blob/master/RE
 
 In this repo, we provide a very simple launcher script named [xla_spawn.py](https://github.com/huggingface/transformers/tree/master/examples/xla_spawn.py) that lets you run our example scripts on multiple TPU cores without any boilerplate.
 Just pass a `--num_cores` flag to this script, then your regular training script with its arguments (this is similar to the `torch.distributed.launch` helper for torch.distributed). 
+Note that this approach does not work for examples that use `pytorch-lightning`.
 
 For example for `run_glue`: