Ignore change of directories or files in chart directory

[zhan9san]

Hi,

Thanks for your great work in this action.

Would it be possibe to ignore change of directories or files in chart directory?

chart-releaser-action/cr.sh

Lines 257 to 258 in db5211e

	local changed_files
	changed_files=$(git diff --find-renames --name-only "$commit" -- "$charts_dir")

Generally, there are README.md, .helmignore in chart directory. If we update them, we don't need to release a new version.

We can define an environment CR_SKIP_EXISTING as a workaround.

Here is an example, prometheus-msteams/prometheus-msteams#320