doc: remove installing from source

also add "run from source" in contributing.md

Author: xxchan

Contributing.md

@@ -91,6 +91,16 @@ Please try to avoid leaving `TODO`s in the code. There are a few around, but I
 wish there weren't. You can leave `FIXME`s, preferably with an issue number.
 
 
+### Run Rustfmt from source code
+
+You may want to run a version of rustfmt from source code as part of a test or
+hacking on the rustfmt codebase. It's strongly discouraged to install a version
+of rustfmt from source. Instead, run it using `cargo run`, and `--manifest-path`.
+
+```
+cargo run --bin cargo-fmt -- --manifest-path path/to/project/you/want2test/Cargo.toml
+```
+
 ### Version-gate formatting changes
 
 A change that introduces a different code-formatting should be gated on the

README.md

@@ -71,24 +71,6 @@ because in the future Rustfmt might work on code where it currently does not):
   fixes to break our stability guarantees).
 
 
-## Installation
-
-```sh
-rustup component add rustfmt
-```
-
-## Installing from source
-
-To install from source (nightly required), first checkout to the tag or branch you want to install, then issue
-
-```sh
-cargo install --path .
-```
-
-This will install `rustfmt` in your `~/.cargo/bin`. Make sure to add `~/.cargo/bin` directory to
-your PATH variable.
-
-
 ## Running
 
 You can run Rustfmt by just typing `rustfmt filename` if you used `cargo