Added current pet peeves abstract

current pet peeves in dotnet.md

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+C# has a lot of history, and went through a lot of changes throughout. The good, the bad, and the ugly ones. Some changes changed the way we write code, some just bolted new ways on top, leaving the old ways to fester.
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+We will talk about the latter. About things that should've been removed, hidden, or otherwise deprecated, but weren't. And nowadays, they create a potent trap for any newcomers to the language. After all, if I was told to use `ArrayList` in Java, why would I not use `ArrayList` in C# as well? It's surely the same thing, and the IDE doesn't complain!
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+We'll take a look at the old and the new, compare, benchmark, and complain about the lack of `[Deprecated]` on the `HashTable` class.