Add a section on how calling the dokku binary should work

[ci skip]

docs/appendices/0.6.0-migration-guide.md

@@ -1,6 +1,7 @@
 # 0.6.0 Migration Guide
 
 ## zero-downtime deployment
+
 - You can now **actually** disable zero-downtime deployments per-app and per-process-type
   - Some config variables have been moved around
     - `DOKKU_CHECKS_ENABLED` has been migrated to `DOKKU_CHECKS_SKIPPED`
@@ -9,5 +10,17 @@
   - See the [updated `checks`](/dokku/checks-examples/) docs for more info
 
 ## proxy port mapping
+
 - You can now configure host -> container proxy port mappings
   - The UI is handled by the [proxy](/dokku/proxy/) interface plugin by setting the `DOKKU_PROXY_PORT_MAP` config variable in the format of `scheme:host-port:container-port`. The default `nginx-vhosts` proxy plugin supports both the `http` and `https` schemes.
+
+## calling the dokku binary
+
+- Plugins should *not* call the dokku binary directly. Clients using the `--app` argument are potentially broken, amongst others, when doing so. Instead, please source the `functions` file for a given plugin when attempting to call dokku internal functions. As a result, the following dokku commands are no longer publicly exposed:
+ - dokku build
+ - dokku receive
+ - dokku release
+ - dokku tar:build
+ - dokku tar:build-locked
+ - dokku git:build
+ - dokku git:build-locked