Proposed GitHub issue template for new XLIFF spec change proposals

["Yoshito Umaoka" <yoshito_umaoka@us.ibm.com>]

Dear all,

In the TC call
today, we discussed about GitHub issue template for XLIFF spec change proposal.

I created a test
repository and experimented the new issue template with YAML.

https://github.com/yumaoka/test-repo/issues/new/choose

Please take a
look if the form looks good for change proposal creation. A newly created
issue will have a label "change proposal" set automatically.
If this is good
enough, I'll create a PR to the XLIFF spec repository.

Thanks,
Yoshito