Only log skipped files in verbose mode

[talal]

This PR changes the default behavior of logging all skipped files to instead logging the skipped files only in verbose mode (with -v flag).

Motivation

When a ton of files are skipped (e.g. -ignore vendor/**) then the output of addlicense becomes quite verbose and the actual output which we are interested in, i.e. which files were modified or which files are missing license info when -check is used, gets buried in between the skipping log lines.

P.S. I have also updated the usage instructions in the README with the up-to-date flag information as per addlicense --help.

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[talal]

@willnorris can you have a look at the PR? I have already signed the CLA.

Feel free to close it, if you don't want to change the app behavior :)

[braydonk]

+1 for this! Really hope this gets merged 🤞

[willnorris]

Sorry, I missed seeing this when it was originally submitted, but yeah this seems totally reasonable.