Add change note announcing generics support

ql/lib/change-notes/2022-05-11-generics-support.md

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+category: feature
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+* Go 1.18 generics are now extracted and can be explored using the new CodeQL classes `TypeParamDecl`, `GenericFunctionInstantiationExpr`, `GenericTypeInstantiationExpr`, `TypeSetTerm` and  `TypeSetLiteralType`, as well as using new predicates defined on the existing `InterfaceType`. Class- and predicate-level documentation can be found in the [Go CodeQL library reference](https://codeql.github.com/codeql-standard-libraries/go/).