Add a configuration file but explicitly ignore all source files. The aim
of this is that we can then un-ignore files a few at a time and deal
with issues in a small isolated manner.
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
Tobin C. Harding