Log what actions are performed

[dscho]

There is a relatively recent issue that results in duplicate issues being opened, e.g. git-for-windows/git#5887.

At first, I thought that it may be a 429 that is not handled properly. But that should have been addressed by #99, and that fix was merged and released before above-mentioned issue was opened.

The fact that I cannot see what is going wrong from looking at the log strongly suggests that this Action needs to output more:

• It needs to print a ::notice:: when it opened a new issue
• It needs to print out which new RSS feed entries it wants to look for
• For the time being, it might need to print out which existing issues it saw
• In general, the Action should log what it is doing (retrieving feed from <url>, looking through <N> feed items, etc)

Slightly edited from #99 (comment)