Add a disabled rustfmt.toml

We do not currently use `rustfmt`, this is a nuisance for devs who
routinely work on code bases that do use rustfmt who often have their
editors set to format on save. We can make the life of such devs much
better by explicitly disabling formatting using `rustfmt.toml`.

ref: https://rust-lang.github.io/rustfmt/?version=v1.4.38&search=#disable_all_formatting

Author: tcharding

rustfmt.toml

@@ -0,0 +1 @@
+disable_all_formatting = true