Add an issue template for developers to support advancing a feature

[jyasskin]

To advance a feature to the next stage of the standards process, we need developers to endorse the feature. It'd be nice to give them guidance about how to do that effectively, especially to explain how their users benefit from the feature. It sounds like issues in the repository are an effective way to let arbitrary developers express support, and issue templates are good ways to guide people to write useful issues. So let's add one here that'll get picked up by future explainer repos.

[b1tr0t]

🙏 this would be very helpful, thank you. I think this could significantly improve the quality of developer feedback on features, especially in helping them to focus on describing the user problems they are trying to solve.