Add reproducible example to LRUStoreCache docstring

[notmatthancock]

The example given for LRUStoreCache is helpful, but unfortunately not run-able since it depends on a remote connection. I came up with the following example that uses a dummy "slow dict" store to be used instead as an example.

However, the factor of 5 on max_size was done through guess-work and should be set to something more appropriate for illustration.

class SlowDictStore(zarr.DictStore):
    def __getitem__(self, item):
        if not item.endswith('.zarray'):
            import time
            time.sleep(1)
        return super().__getitem__(item)

x = np.zeros(128)

store = SlowDictStore()
cache = zarr.LRUStoreCache(store=store, max_size=x.nbytes*5)
root = zarr.group(store=cache)

root.create_dataset('x', data=x.copy(), compressor=None)
root.create_dataset('y', data=x.copy(), compressor=None)

cache.invalidate()

from timeit import timeit
print('1st access x:', timeit('root["x"][:]', number=1, globals=globals()))
print('2nd access x:', timeit('root["x"][:]', number=1, globals=globals()))

print('1st access y:', timeit('root["y"][:]', number=1, globals=globals()))
print('2nd access y:', timeit('root["y"][:]', number=1, globals=globals()))