Add missing pre-requisite to build WASM client

[jpraynaud]

Why

We have noticed that the pre-requisites to setup the WASM client build tools is missing the setup of the Rust target:

• wasm32-unknown-unknown target is missing:

warning: blst@0.3.11: error: unable to create target: 'No available targets are compatible with triple "wasm32-unknown-unknown"'
warning: blst@0.3.11: 1 error generated.

• wasm32-unknown-emscripten is missing with emcc:

warning: blst@0.3.11: emcc: error: unsupported target: wasm32-unknown-unknown (emcc only supports wasm64-unknown-emscripten and wasm32-unknown-emscripten)

error: failed to run custom build command for `blst v0.3.11`

What

Add the commands to setup the Rust target(s) in the documentation of the WASM client

How

• Add target rustup target add wasm32-unknown-unknown with default pre-requisites
• Add target rustup target add wasm32-unknown-emscripten with emcc pre-requisites