Add 'For anyone else' reminder about adding reactions

[wilzbach]

An idea from https://github.com/ansible/ansibullbot/blob/master/ISSUE_HELP.md#for-anyone-else

[dlang-bot]

Thanks for your pull request, @wilzbach!

[wilzbach]

CC @WebDrake as you helped to shape the initial message and are generally interested in the bot.

[WebDrake]

Looks like a nice addition to me! I'd just keep an eye on behaviour to make sure that it doesn't result in lots of 👍 comments (instead of the wanted reactions).

I'd sound a note of caution about quite how informative these metrics will be though. Likely they will reflect more the preferences and interests of the people hanging round the dlang GitHub, more than the preferences and interests of the wider user community.

[MoonlightSentinel]

Not sure if the call for reactions is required but I like the suggestion to review even as a new/normal contributer.

[CyberShadow]

I think at this point this is part of well-known GitHub culture, is this still worth the vertical space?