[Hexagon] Add release notes for 6.0.0
[Hexagon] Add release notes for 6.0.0


git-svn-id: aad6896a0674cbe2af7806761e473047ca8ef14a@324248 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm