Add ExtraCalldepth to Logger

ExtraCalldepth is added to the default calldepth of 2 to allow the
correct function name to be shown when exposing logger methods at
package level or wrapping them in utility functions

Add an example of how this works to examples/example.go
Update examples/example.go to include a new screenshot
Update README.md to include the new example.go
I thought I'd note that this change was caused by me copying the updated contents of example.go directly into the code block; it looks like it was updated in an earlier commit but not amended in the code block in README.md