Call out semver expectations for modules (apache#3659)

* Call out semver expectations for modules

* Update modules.md

* Link to versioning

docs/content/development/modules.md

@@ -8,7 +8,7 @@ Druid uses a module system that allows for the addition of extensions at runtime
 
 ## Writing your own extensions
 
-Druid's extensions leverage Guice in order to add things at runtime.  Basically, Guice is a framework for Dependency Injection, but we use it to hold the expected object graph of the Druid process.  Extensions can make any changes they want/need to the object graph via adding Guice bindings.  While the extensions actually give you the capability to change almost anything however you want, in general, we expect people to want to extend one of a few things.
+Druid's extensions leverage Guice in order to add things at runtime.  Basically, Guice is a framework for Dependency Injection, but we use it to hold the expected object graph of the Druid process.  Extensions can make any changes they want/need to the object graph via adding Guice bindings.  While the extensions actually give you the capability to change almost anything however you want, in general, we expect people to want to extend one of the things listed below.  This means that we honor our [versioning strategy](./versioning.html) for changes that affect the interfaces called out on this page, but other interfaces are deemed "internal" and can be changed in an incompatible manner even between patch releases.
 
 1. Add a new deep storage implementation
 1. Add a new Firehose