Update the rust source code download method in docs

[DavidBM]

Actually you can do with rustup rustup component add rust-src and then get the source path with rustc --print sysroot. The source code will be in: <toolchain root>/lib/rustlib/src/rust.

I think this way is more reliable for the user and the pluggin. Moreover, this allow to change version with rustup and update the source code automatically.

More information:
rust-lang/rustup#37 (comment)
racer-rust/racer#595

[DavidBM]

For now I'm using this in my .profile (ubuntu 16.10)

export RUST_SRC_PATH="$(rustc --print sysroot)/lib/rustlib/src/rust"