Typo: usefull -> useful

Author: enigmata

README.md

@@ -36,7 +36,7 @@ Gather and analyse crash information.
 
 We use Incredibuild to make use of all cores of multiple machines to accelerate building our C++ projects.
 
-So when is this Debugview++ thing usefull?
+So when is this Debugview++ thing useful?
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 - first of all, with debugview++ you can see messages from different processes, not just 'attached' processes.