Merge pull request #2017 from Xe/docs/deisctl/custom-units

docs(deisctl): document DEISCTL_UNITS better

Author: mboersma

deisctl/README.md

@@ -46,6 +46,10 @@ Then, export the `DEISCTL_UNITS` environment variable so deisctl can find the un
 $ export DEISCTL_UNITS="$PATH_TO_DEISCTL/units"
 ```
 
+This is also useful for specifying custom behavior on Deis units, such as using 
+fleet metadata to lock the builder to a more powerful node, or keep application 
+nodes free of control plane elements.
+
 ## Remote Configuration
 
 While `deisctl` can be used locally on a CoreOS host, it is extremely useful as a tool