A README from a parent directory isn't added to the pkg

[Michael-F-Bryan]

🐛 Bug description

When using workspaces it's common to have a directory hierarchy like this:

• /
  • Cargo.toml
  • README.md
  • my-wasm-code/
    • Cargo.toml
    • src/
    • pkg/
  • ...

And the my-wasm-code crate will use the package.readme field in Cargo.toml to tell cargo where to find the README.

[package]
name = "aimc_sim"
version = "0.1.0"
authors = ["Michael Bryan <michaelfbryan@gmail.com>"]
edition = "2018"
description = "WASM frontend for the Adventures in Motion Control simulator"
license = "MIT or Apache-2.0"
repository = "https://github.com/Michael-F-Bryan/adventures-in-motion-control"
publish = false
readme = "../README.md"

At the moment wasm-pack doesn't follow this convention, and will just look in the crate's directory for a README file.

🤔 Expected Behavior

wasm-pack should check the Cargo.toml for a readme field, before falling back to crate_dir.join("README.md") when adding a README.md to the pkg/ directory.

I'm guessing this is the relevant area of code? The naive solution (parse Cargo.toml and try to use the filename from package.readme, if provided) would probably be good enough, but I'm not sure whether this sort of work should be handled elsewhere in the application.

👟 Steps to reproduce

$ mkdir /tmp/repro && cd /tmp/repro

$ cat Cargo.toml
[workspace]
members = ["my-wasm-code"]

$ cat README.md 
# Minimal Repro

$ cargo generate --git https://github.com/rustwasm/wasm-pack-template --name my-wasm-code
 Creating project called `my-wasm-code`...
 Done! New project created /tmp/repro/my-wasm-code

$ ls my-wasm-code
Cargo.toml   src  tests

$ head my-wasm-code/Cargo.toml 
[package]
name = "my-wasm-code"
version = "0.1.0"
authors = ["Michael Bryan <michaelfbryan@gmail.com>"]
edition = "2018"
readme = "../README.md"

$ wasm-pack build my-wasm-code 
[INFO]: Checking for the Wasm target...
[INFO]: Compiling to Wasm...
...
[INFO]: :-) Done in 2m 41s
[INFO]: :-) Your wasm pkg is ready to publish at my-wasm-code/pkg.

$ ls pkg 
my_wasm_code.d.ts  my_wasm_code.js  my_wasm_code_bg.d.ts  my_wasm_code_bg.wasm  package.json

🌍 Your environment

$ wasm-pack --version
wasm-pack 0.8.1
$ rustc --version
rustc 1.39.0-nightly (6ef275e6c 2019-09-24)