[CONTRIBUTING.md] Add section about merging ECS conventions

CONTRIBUTING.md

@@ -180,4 +180,21 @@ make misspell-correction
   - Send staging tag as PR for review.
 - Create a tag `v{version}` on the merged PR and push remote.
 
+## Merging existing ECS conventions
+
+The Elastic Common Schema (ECS) is being merged into OpenTelemetry Semantic
+Conventions per [OTEP 222][otep222]. When adding a semantic convention that
+exists in some form in ECS, consider the following guidelines:
+
+- Prefer using the existing ECS name when possible. In particular:
+  - Do not amend ECS names to follow plural guidelines or other soft
+    recommendations.
+  - If proposing a name that differs from the ECS convention, provide usage
+    data, user issue reports or feature requests or comparable evidence about
+    the alternatives.
+- Do not use an existing ECS name as a namespace. If the name must differ, use a
+  different namespace name to avoid clashes or avoid using the namespace
+  entirely.
+
 [nvm]: https://github.com/nvm-sh/nvm/blob/master/README.md#installing-and-updating
+[otep222]: https://github.com/open-telemetry/oteps/pull/222