Update Readme to use the init.sh script (paritytech#243)

README.adoc

@@ -126,10 +126,6 @@ If you'd actually like hack on Polkadot, you can just grab the source code and b
 [source, shell]
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 curl https://sh.rustup.rs -sSf | sh
-rustup update nightly
-rustup target add wasm32-unknown-unknown --toolchain nightly
-rustup update stable
-cargo install --git https://github.com/alexcrichton/wasm-gc
 sudo apt install cmake pkg-config libssl-dev git clang
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@@ -145,6 +141,7 @@ Then build the code:
 
 [source, shell]
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+./scripts/init.sh # Install WebAssembly. Update Rust
 ./scripts/build.sh # Builds the WebAssembly binaries
 cargo build # Builds all native code
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