site: Explain incompletes and aborts

They seem to confuse people and the severity is not obvious immediately.
Let's put that in plain text.

Issue: #92Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>

site/pre-merge-results.md

@@ -91,6 +91,20 @@ Each test state change (e.g. pass→fail) has few things listed:
  * **numer of occurences of same transition across multiple machines:** `+3 similar issues`
 
 
+## How Serious Are Incompletes And Aborts?
+
+**Incomplete** means that the machine haven't been able to complete execution
+of a test (most often because of a kernel panic or a hard hang). Incompletes
+signal very serious issues with stability. Any tests that were supposed to
+follow, that are on the same testlist (shard, fast-feedback), are not going to
+be executed.
+
+**Aborts** are as serious as incompletes but the abnormal state was detected
+either by IGT Runner or the test itself. They signal that no further testing
+is going to be done because the results would not be trustworthy do to the
+machine's / kernel condition.
+
+
 ## I've Got Some False Positives, What Now?
 
 Reply to the e-mail with the results and explain why do those results are