Adding Pull Request template

[chemicL]

With the template, contributors can learn our conventions and strategy for squash merge commit so that the PR description is properly formatted.

[violetagg]

@chemicL Are you targeting only Reactor Core or Project Reactor? If it is the latter you need to add this on the Project Reactor level here https://github.com/reactor/.github/tree/main/.github/ISSUE_TEMPLATE

[violetagg]

The title is used in the release notes so it is not that good to have this on the title level.
Previously we agreed to have this only on commit level.

[chemicL]

Thanks, that slipped. We'll just need to remember when merging.

[chemicL]

@chemicL Are you targeting only Reactor Core or Project Reactor? If it is the latter you need to add this on the Project Reactor level here https://github.com/reactor/.github/tree/main/.github/ISSUE_TEMPLATE

Just Reactor Core. If we find this approach useful and other projects would like to benefit from it, we can always move this template to the broader configuration.