docs: add proposal template for new contributionsdocs: add proposal template (fixes #4)

PROPOSAL.md

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+# Neuroscience Data Structure Proposal Template
+
+Thank you for contributing to the Neuroscience Data Structure project!  
+Please use the following template to describe your proposed changes or additions.  
+
+---
+
+## 1. Proposal Title
+_A short descriptive title for your proposal._
+
+---
+
+## 2. Summary
+_A brief overview (2–3 sentences) of the proposal and its purpose._
+
+---
+
+## 3. Motivation
+_Why is this change needed? What problem does it solve for the neuroscience data community?_
+
+---
+
+## 4. Detailed Description
+_Provide a clear explanation of the proposed feature, format, or improvement. Include technical details, workflows, or examples if applicable._
+
+---
+
+## 5. Alternatives Considered
+_Have you considered other options? Why is this proposal the best approach?_
+
+---
+
+## 6. Expected Impact
+_What are the benefits of this proposal? Are there potential risks or limitations?_
+
+---
+
+## 7. Example Use Case
+_Provide a practical example or scenario where this proposal would be useful._
+
+---
+
+## 8. Additional Notes
+_Any references, related work, links, or further information that supports your proposal._
+
+---
+
+## 9. Checklist
+- [ ] I have checked existing proposals to avoid duplication  
+- [ ] I have provided sufficient detail for reviewers to evaluate this proposal  
+- [ ] I am willing to revise this proposal based on community feedback  
+"# Proposed Directory Structure and Naming Conventions"