add convention for plugin location (relates to #13879)

Signed-off-by: Amitai Stern <[email protected]>

CONVENTIONS.md

@@ -15,6 +15,12 @@
   - [Identifiers](#identifiers)
   - [Variables](#variables)
 
+## Plugin Location
+
+- Plugins should be created in their own external repository. External to the OpenSearch Project itself. 
+- Plugins can be later ported into the OpenSearch Project on a case-by-case basis. A formal path for this process is yet to be created.
+- Do not add new plugins to the [plugins folder](https://github.com/opensearch-project/OpenSearch/tree/main/plugins) within the project. This folder is within the core project, where the maintainers are not focussed on optional pluggable features. If you think it should be there anyway, please explain why in the RFC stage. In any case, it is recommended to start outside the project.   
+
 ## Plugin Naming Conventions
 
 ### GitHub