[llvm] [Offload][NFC] Add offload subfolder and README (PR #77154)

[Shilei Tian via llvm-commits]

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+# The LLVM/Offload Subproject
+
+The Offload subproject aims at providing tooling, runtimes, and APIs that allow
+users to execute code on accelerators or other "co-processors" that may or may
+not match the architecture of their "host". In the long run, all kinds of
+targets are in scope of this effort, including but not limited to: CPUs, GPUs,
+FPGAs, AI/ML accelerators, distributed resources, etc.
+
+The project is just starting and the design is still not ironed out. More
+content will show up here and on our webpage soon. In the meantime people are
+encouraged to participate in our meetings (see below) and check our
+[development board](https://github.com/orgs/llvm/projects/24/) as well as the
+discussions on [discord](https://discourse.llvm.org/tag/offload).
----------------
shiltian wrote:

```suggestion
discussions on [Discourse](https://discourse.llvm.org/tag/offload).
```

https://github.com/llvm/llvm-project/pull/77154