doc: Link to Capability Levels documentation (#2660)

Author: dmesser

README.md

@@ -154,6 +154,8 @@ Note that each operator type has a different set of capabilities. When choosing
 
 ![operator-capability-level](./doc/images/operator-capability-level.png)
 
+Find more details about the various levels and the feature requirements for them in the [capability level documentation][capability_levels].
+
 ## Samples
 
 To explore any operator samples built using the operator-sdk, see the [operator-sdk-samples][samples].
@@ -199,3 +201,4 @@ Operator SDK is under Apache 2.0 license. See the [LICENSE][license_file] file f
 [helm_user_guide]:./doc/helm/user-guide.md
 [faq]: ./doc/faq.md
 [getting_started]: https://github.com/operator-framework/getting-started/blob/master/README.md
+[capability_levels]: ./doc/operator-capabilities.md