[clang] [Clang] [Contracts] Design Doc (PR #229978)

[Kazu Hirata via cfe-commits]

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+# Introduction
+
+This is a high level design documentation for Contracts implementation in Clang.
+This summarized experiences from several implementors in seperate branches.
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kazutakahirata wrote:

```suggestion
This summarized experiences from several implementors in separate branches.
```

https://github.com/llvm/llvm-project/pull/229978