Add shim/README.md to explain what the shim/ tree is for

It's not obvious why this tree is here. Add a README explaining it.

Author: dbort

shim/README.md

@@ -0,0 +1,16 @@
+# buck2 shims
+
+The `shim/` subtree helps the Meta-internal buck2 build system also work in the
+open-source repo.
+
+Shims are how open-source buck2 supports a [line
+like](https://github.com/pytorch/executorch/blob/50aa517549d10324147534d91d04a923b76421d6/kernels/optimized/targets.bzl#L1):
+
+```
+load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
+```
+
+In the open-source repo, `fbsource//xplat` (a Meta-internal root) doesn't exist.
+The `shim = shim` line in `../.buckconfig` tells buck2 to look in
+[`shim/xplat/executorch/build/runtime_wrapper.bzl`](https://github.com/pytorch/executorch/blob/main/shim/xplat/executorch/build/runtime_wrapper.bzl)
+instead.