Add README

Author: Vinh Tran

examples/toolchain-to-rebuild-std/.bazelrc

@@ -4,9 +4,7 @@ build --@rules_rust//rust/settings:experimental_toolchain_generated_sysroot=True
 # enable toolchain debugging by default
 build --toolchain_resolution_debug='@rules_rust//rust:toolchain_type'
 
+# for toolchain debugging purposes
 build:beta --@rules_rust//rust/toolchain/channel:channel=beta
 build:stable --@rules_rust//rust/toolchain/channel:channel=stable
 build:nightly --@rules_rust//rust/toolchain/channel:channel=nightly
-
-build:apple --platforms=//:aarch64-apple-darwin
-build:wasi --platforms=//:wasm32-wasi

examples/toolchain-to-rebuild-std/README

@@ -1,6 +1,34 @@
 
+This directory demonstrates an example of how to rebuild the standard library
+for an arbitrary platform and custom configurations.
+
+TODO: Write up on how the toolchain bootrapping process works.
+
+To build an end-user rust_library target for specific target, run
+
+```
+bazel build //hello_lib --platforms=//:aarch64-apple-darwin
+```
+
+for `aarch64-apple-darwin` or
+
+```
+bazel build //hello_lib --platforms=//:wasm32-wasi
+```
+
+for `wasm32-wasi`.
+
+The toolchains orderly registered in WORKSPACE and _beta_channel_transition ensure
+that the standard library is rebuilt for the given platform.
+
+To only rebuild std, run
+
+```
+bazel build @stdlbs//:std --platforms=//:aarch64-apple-darwin
+```
+
+or
 
 ```
-bazel build //hello_lib --config=apple
-bazel build //hello_lib --config=wasi
+bazel build @stdlbs//:std --platforms=//:wasm32-wasi
 ```