* docs: Fixes#204
* Add command to switch to a stable version of Rust before installing Polkadot PoC-1 since error occurs when trying to install using Rust Nightly, which some users may have configured as their active toolchain
* Append ` polkadot subkey` to the command that installs the latest version of Polkadot PoC-1 to overcome error
* Update README.md