Add contributing guidelines

.prettierrc.json

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+{
+  "overrides": [
+    {
+      "files": "*.md",
+      "options": {
+        "proseWrap": "always"
+      }
+    }
+  ]
+}

CONTRIBUTING.md

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+# Contributing Guidelines
+
+We're happily accepting bugfixes, style improvements, good refactors and enhancements to documentation.
+
+For the specs, their APIs are considered final and immutable. We do not accept API-breaking changes.
+
+It is possible to propose a new protocol instead. In that case, please open an issue for discussion first, including
+some motivating use cases.
+
+The contracts are not expected to be packed with features. They're expected to be minimal, reference implementations of
+the specifications. We do not therefore accept enhancements.