Auto merge of #7460 - camsteffen:run-from-source, r=Manishearth
Auto merge of #7460 - camsteffen:run-from-source, r=Manishearth

Add instructions to run from source

changelog: none

We often get messages on Zulip asking how to install and run Clippy from source. This adds instructions to the readme. I also added a note explaining that `cargo install --path . --force` is bad, which I just decided after some investigating. I use macOS. It would be nice to get some tests on other platforms to see if this is correct.

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust