Add docs about weirdness

Author: adamtheturtle

CONTRIBUTING.rst

@@ -176,6 +176,8 @@ However, the maximum allowed value is 50.
 
 A response to an invalid query may have an ``application/json`` content type but include text (not JSON) data.
 
+After deleting a target, for up to approximately 30 seconds, matching it with a query returns a 500 response.
+
 Performing a release
 --------------------
 

README.rst

@@ -177,6 +177,13 @@ Matching a target which is in the processing state sometimes returns a successfu
 Sometimes a 500 (INTERNAL SERVER ERROR) response is given.
 The mock always gives a 500 response.
 
+Matching deleted targets
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Matching a target which has been deleted returns a 500 (INTERNAL SERVER ERROR) response within the first few seconds.
+This timeframe is not consistent on the real Vuforia Web Services.
+On the mock, this timeframe is always three seconds.
+
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