Update writing_exporters.md (#746)

Add note about choosing port numbers for exporters on internal
applications. This reflects that the port range is a mere suggestion.

Discussion on prometheus-users is in
https://groups.google.com/d/topic/prometheus-users/S1ZzJMkR1tk/discussion

content/docs/instrumenting/writing_exporters.md

@@ -459,7 +459,8 @@ preferably before publicly announcing it. If you’re not ready to release yet,
 putting your username and WIP is fine.
 
 This is a registry to make our users’ lives a little easier, not a commitment
-to develop particular exporters.
+to develop particular exporters. For exporters for internal applications we
+recommend using ports outside of the range of default port allocations.
 
 ## Announcing