Update bazel readme (#774)

* Update bazel/README.md

* Add deps instantiation to readme

* Add bazelrc explanations to readme

* Note the preferred way of using bazel emsdk

Author: celentes

bazel/README.md

@@ -2,40 +2,34 @@
 
 ## 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:
+In `WORKSPACE` file, put:
 ```
-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
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+http_archive(
+    name = "emsdk",
+    strip_prefix = "emsdk-c1589b55641787d55d53e883852035beea9aec3f/bazel",
+    url = "https://github.com/emscripten-core/emsdk/archive/c1589b55641787d55d53e883852035beea9aec3f.tar.gz",
+    sha256 = "7a58a9996b113d3e0675df30b5f17e28aa47de2e684a844f05394fe2f6f12e8e",
+)
+
+load("@emsdk//:deps.bzl", emsdk_deps = "deps")
+emsdk_deps()
+
+load("@emsdk//:emscripten_deps.bzl", emsdk_emscripten_deps = "emscripten_deps")
+emsdk_emscripten_deps()
 ```
 
 ## Building
 
 ### Using --config=wasm
+
+Put the following lines into your `.bazelrc`:
+```
+build:wasm --crosstool_top=//emscripten_toolchain:everything
+build:wasm --cpu=wasm
+build:wasm --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
+```
+
 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.
 
@@ -44,7 +38,7 @@ 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")
+load("@emsdk//emscripten_toolchain:wasm_rules.bzl", "wasm_cc_binary")
 
 cc_binary(
     name = "hello-world",
@@ -61,3 +55,7 @@ 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.
+
+`wasm_cc_binary` uses transition to use emscripten toolchain on `cc_target`
+and all of its dependencies, and does not require amending `.bazelrc`. This
+is the preferred way, since it also unpacks the resulting tarball.