add a wasm bazel toolchain (#603)

bazel/README.md

@@ -0,0 +1,63 @@
+# Bazel Emscripten toolchain
+
+## Setup Instructions
+
+1. Merge the `WORKSPACE` file in with your own at the root of your bazel
+directory structure. If you don't have one, simply copy the file.
+2. Merge the `bazelrc` file in with your `.bazelrc` file at the root of your
+bazel directory structure. If you don't have one, simply copy the file and
+rename it to `.bazelrc`. (Note the `.`)
+3. Copy the `emscripten_toolchain` folder along with its contents to the root of
+your bazel directory.
+
+Your directory structure should look like this:
+```
+bazel_root/
+├── .bazelrc
+├── WORKSPACE
+├── emscripten_toolchain/
+│   ├── BUILD.bazel
+│   ├── builddefs.bzl
+│   ├── crosstool.bzl
+│   ├── emar.sh
+│   ├── emcc.sh
+│   ├── emcc_link.sh
+│   ├── emscripten.BUILD
+│   ├── emscripten_config
+│   ├── env.sh
+│   ├── link_wrapper.py
+│   ├── wasm_binary.py
+│   ├── wasm_cc_binary.bzl
+│   ├── wasm_rules.bzl
+├── your_project_folder/
+│   ├── your_project.file
+```
+
+## Building
+
+### Using --config=wasm
+Simply pass `--config=wasm` when building a normal `cc_binary`. The result of
+this build will be a tar archive containing any files produced by emscripten.
+
+### Using wasm_cc_binary
+First, write a new rule wrapping your `cc_binary`.
+
+```
+load("@rules_cc//cc:defs.bzl", "cc_binary")
+load("//emscripten_toolchain:wasm_rules.bzl", "wasm_cc_binary")
+
+cc_binary(
+    name = "hello-world",
+    srcs = ["hello-world.cc"],
+)
+
+wasm_cc_binary(
+    name = "hello-world-wasm",
+    cc_target = ":hello-world",
+)
+```
+
+Now you can run `bazel build :hello-world-wasm`. The result of this build will
+be the individual files produced by emscripten. Note that some of these files
+may be empty. This is because bazel has no concept of optional outputs for
+rules.