Rust-clippy: Better install instructions for stable

[spease]

Some explicit install instructions for stable rust would be nice. These seem to be the crucial two, although it looks like clippy is broken on nightly right this instant.

rustup install nightly
rustup run nightly cargo install clippy

[spease]

Note that the second command can be simplified to
cargo +nightly install clippy
Although since clippy got fixed, I've been having trouble running it with just cargo clippy. So cargo +nightly clippy can be used to run it too.

[pravic]

Could not install clippy: tons of errors like

error[E0609]: no field int_type on type rustc::session::config::Config

Version:

$ rustc +nightly -vV
rustc 1.22.0-nightly (dd08c3070 2017-09-12)
binary: rustc
commit-hash: dd08c30703d052205a68ae34331eea464178cd99
commit-date: 2017-09-12
host: i686-pc-windows-msvc
release: 1.22.0-nightly
LLVM version: 4.0

$ cargo show clippy
...
max_version: 0.0.159

Tried also 158, 157...

[oli-obk]

That's an independent issue #2047