docs: document that the `pkg` directory is gitignored by default

Fixes #473

docs/src/commands/build.md

@@ -5,6 +5,10 @@ interoperability and for publishing a package to npm. This involves compiling
 your code to wasm and generating a pkg folder. This pkg folder will contain the
 wasm binary, a JS wrapper file, your `README`, and a `package.json` file.
 
+The `pkg` directory is automatically `.gitignore`d by default, since it contains
+build artifacts which are not intended to be checked into version
+control.<sup>[0](#footnote-0)</sup>
+
 ## Path
 
 The `wasm-pack build` command can be given an optional path argument, e.g.:
@@ -85,3 +89,9 @@ wasm-pack build examples/js-hello-world --mode no-install
 |---------------|------------------------------------------------------------------------------------------|
 | `no-install`  | `wasm-pack init` implicitly and create wasm binding  without installing `wasm-bindgen`.  |
 | `normal`      | do all the stuffs of `no-install` with installed `wasm-bindgen`.                         |
+
+<hr style="font-size: 1.5em; margin-top: 2.5em"/>
+
+<sup id="footnote-0">0</sup> If you need to include additional assets in the pkg
+directory and your NPM package, we intend to have a solution for your use case
+soon. [↩](#wasm-pack-build)