Add DIP forum guidelines

Author: mdparker

docs/guidelines-forums.md

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+# DIP Forum Guidelines
+
+The DIP Ideas and DIP Development forums facilitate feedback for D Improvement Proposals (DIPs) from the D community and language maintainers. These forums have stricter moderation than standard D forums.
+
+Posts violating these guidelines may be deleted without warning. Our aim is to maintain focused, purposeful discussions.
+
+Guidelines may change as needed.
+
+## DIP Ideas
+This forum is for floating new DIP ideas or seeking authors to develop them.
+
+Rules for new threads:
+
+1. Focus on a new DIP idea or a variation of an existing one.
+2. Provide a detailed description of the idea within the post for informed feedback. Links for additional context are acceptable.
+3. Titles should clearly state the core idea, avoiding unnecessary words. For example, use "A variation on sumtypes" or "Tuples v2" instead of vague titles like "Looking for feedback" or "What do you think about this tuple idea?".
+
+## DIP Development
+This forum is for discussing drafts of DIPs.
+
+Rules for new threads:
+
+1. Focus on a specific draft of a DIP (e.g., first draft, second draft).
+2. Include a permanent GitHub link to the draft under discussion.
+3. For the first draft, summarize the DIP; for subsequent drafts, summarize revisions.
+4. Format titles as "First|Second|Third|N Draft: DIP Title", like "First Draft: Tuples".
+
+## Feedback
+Feedback in both forums must:
+
+1. Be on-topic, relating directly to the DIP idea or draft.
+2. Be respectful and constructive.
+3. Provide context and useful information. Avoid short, contextless remarks like "LGTM" or "Thumbs up!" Explain your reasoning.
+
+In the DIP Ideas forum, there is more leeway for off-topic discussion to refine ideas.
+
+In the DIP Development forum, feedback should be actionable, helping authors refine their proposals. Express opinions on a feature in the DIP Ideas forum; provide constructive improvement suggestions in the DIP Development forum.