Document current limitations of loop (#1996)

* Document current limitations of loop

* Update README.md

plugin/loop/README.md

@@ -2,7 +2,7 @@
 
 ## Name
 
-*loop* - detect forwarding loops and halt the server.
+*loop* - detect simple forwarding loops and halt the server.
 
 ## Description
 
@@ -13,7 +13,7 @@ The plugin will try to send the query for up to 30 seconds. This is done to give
 to start up. Once a query has been successfully sent *loop* disables itself to prevent a query of
 death.
 
-The query send is `<random number>.<random number>.zone` with type set to HINFO.
+The query sent is `<random number>.<random number>.zone` with type set to HINFO.
 
 ## Syntax
 
@@ -38,3 +38,10 @@ After CoreDNS has started it stops the process while logging:
 ~~~ txt
 plugin/loop: Seen "HINFO IN 5577006791947779410.8674665223082153551." more than twice, loop detected
 ~~~
+
+## Limitations
+
+This plugin only attempts to find simple static forwarding loops at start up time.  To detect a loop, all of the following must be true
+
+* the loop must be present at start up time.
+* the loop must occur for at least the `HINFO` query type.