otlp exporter readme appears to be out of date

[pmcollins]

Hi!

I wanted to experiment with the code listed here but found that it doesn't compile for me.

The line

	pusher := push.New(simple.NewWithInexpensiveDistribution(), exporter)

has the error Cannot use 'simple.NewWithInexpensiveDistribution()' (type export.AggregatorSelector) as type export.Checkpointer Type does not implement 'export.Checkpointer'. I think I found the solution by looking at test code but thought I should add this to track updating the readme.

Thanks!