Update README.md

[nicarq]

Fixes compilation issue when running this instructions:

git submodule update --init --recursive && \
cd js-cardano-wasm && \
npm install && \
./build && \
npm link && \
cd .. && \
npm link rust-cardano-crypto

With this error:

> icarus-light-cardano-wallet-poc@0.0.1 build-js-cardano-wasm /Users/narqueros/emurgo/code/icarus-poc
> ./setup_cardano_crypto.sh

npm WARN rust-cardano-crypto@0.1.2 No license field.

up to date in 2.644s
~/emurgo/code/icarus-poc/js-cardano-wasm/wallet-wasm ~/emurgo/code/icarus-poc/js-cardano-wasm
error: toolchain 'nightly-2018-06-05-x86_64-apple-darwin' is not installed
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! icarus-light-cardano-wallet-poc@0.0.1 build-js-cardano-wasm: `./setup_cardano_crypto.sh`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the icarus-light-cardano-wallet-poc@0.0.1 build-js-cardano-wasm script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/narqueros/.npm/_logs/2018-07-03T21_05_43_173Z-debug.log

(the latest nightly is 2018-06-28)

[SebastienGllmt]

Note: Probably better to add rustup show as the last step to resolve this issue instead of adding the nightly version manually.

[NicolasDP]

Hi,

Indeed, we have set a rust toolchain version here. This was necessary to avoid inconsistent behaviour between consumers of this library using different nightly versions.

Thanks for the change.