Mention how RLS can be built without clippy_lints

[Xanewok]

Related issue - #1260.

Sometimes recent nightlies ship with internal changes that prevent the current clippy_lints dependency to build.

It'd be good to mention that in the contributing docs that

1. one can often build with --no-default-features (disabling the clippy feature)
2. A great help would be to see if the clippy dependency has been updated in the Rust tree (clippy submodule at https://github.com/rust-lang/rust/tree/master/src/tools) and if so, submit a PR here updating the clippy_lints dependency to the git revision pulled from the Rust tree, unblocking the development with clippy.

[rcjsuen]

Should this be closed now that #1267 has been merged?

[Xanewok]

Yeah, thanks!