Include GitHub Issues as an option for feedback

In addition to encouraging pull requests, also suggest that users create
issues for items they encounter that they do not have the time or
wherewithal to propose a fix for themselves.

This would require that GitHub Issues be enabled on the bitcoinbook
repository; at present they are not.

Note that this change introduces inline links for convenience and
readability with the expectation that the affected section ("Early
Release Notes") will be removed before going to press.