Add contribution-welcome label to package-request issue template

[not522]

Contributor Agreements

Please read the contributor agreements and if you agree, please click the checkbox below.

• I agree to the contributor agreements.

Important

The following formatting is a requirement to merge this PR:

$ pip install pre-commit
$ pre-commit run --all-files

Please also try the following to make sure that your module can be loaded from the registry:

import optunahub

module = optunahub.load_module(
    # category is one of [pruners, samplers, visualization].
    package="<category>/<your_package_name>",
    repo_owner="<your_github_id>",
    ref="<your_branch_name>",
)

For more detail, please check the tutorial.

Motivation

package-request issues are almost always contribution-welcome.

Description of the changes

This PR adds the contribution-welcome label to the package-request issue template.

I checked the behavior on my fork repository. https://github.com/not522/optunahub-registry

[y0z]

LGTM