docs: udpate readme - Githubissues

[x] It's the right issue type on the title
[ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
[x] The commit message follows our guidelines
[ ] Tests for the changes have been added (for bug fixes/features)
[x] Docs have been added/updated (for bug fixes/features)
[x] It does not introduce a breaking change or has a description of the breaking change

Thank you for your contribution to TOAST UI product. 🎉 😘 ✨

nhn / toast-ui.vue-calendar