docs: Fixes #204. Update Readme. Add command to switch to Rust stable. Append binaries to installation command (#205)

* 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