Add note on HDF5 file locking

By default you can't open the dump files in two programs at once, which prevents you from post-processing a case which is already running, and can cause an error if you try to re-open a file in a Jupyter Notebook. This can be resolved by setting the following in .bashrc:

`HDF5_USE_FILE_LOCKING=False`

We should add this to the docs/examples.