Add a small README.md for wit-bindgen-rt (#1162)

Requested in rust-lang/rust#136395 (comment)

crates/guest-rust/rt/Cargo.toml

@@ -6,7 +6,7 @@ repository = { workspace = true }
 license = { workspace = true }
 homepage = 'https://github.com/bytecodealliance/wit-bindgen'
 description = """
-Runtime support for the `wit-bindgen` crate
+Internal runtime support for the `wit-bindgen` crate.
 """
 
 [dependencies]

crates/guest-rust/rt/README.md

@@ -0,0 +1,22 @@
+# `wit-bindgen-rt`
+
+This is an internal implementation detail of the [`wit-bindgen`] crate. The
+source for this crate lives at https://github.com/bytecodealliance/wit-bindgen
+and this crate is located in `crates/guest-rust/rt` folder. The purpose of this
+crate is to contain "runtime" code related to the macro-expansion of the
+`wit_bindgen::generate!` macro such that the `wit-bindgen` crate dependency can
+be removed in some situations.
+
+This crate contains a precompiled object file and archive at
+`src/cabi_realloc.o` and `src/libwit_bindgen_cabi_realloc.a`. This is compiled
+from the source `src/cabi_realloc.c` and is checked in as precompiled to avoid
+needing a C compiler at compile-time which isn't always available. This object
+file is only used on wasm targets.
+
+The object file is compiled by
+[this script]https://github.com/bytecodealliance/wit-bindgen/blob/main/ci/rebuild-libcabi-realloc.sh)
+and is verified in repository continuous integration that the checked-in
+versions match what CI produces.
+
+[`wit-bindgen`]: https://crates.io/crates/wit-bindgen
+