Added RFC for style checking with rustfmt

Author: korken89

rfcs/0207-rustfmt-our-libs.md

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+# Summary
+[summary]: #summary
+
+This RFC proposes to use CI to give errors, or bors to apply rustfmt on merge to
+make sure our lib do not diverge in style within the WG's libraries. Moreover,
+it can act as a reference for newcomers to also follow the same style.
+
+# Motivation
+[motivation]: #motivation
+
+The main motivation is to keep the WG's style consistent. As libraries continue
+to evolve over time, as will the style if we do not have a system to keep us in
+check. It could also be used as an example to motivate the newcomers of embedded
+Rust to use `rustfmt` and keep the style consistent. It helps everyone when
+opening someones code and feeling at home, and not needing to reread the code to
+understand the style of the writer, which is a common issue in for example C++.
+
+# Detailed design
+[design]: #detailed-design
+
+All in all, two main methods were discussed:
+
+1. Enforce `rustfmt` at CI level, and have an extra target for formating which
+will fail if the code is not following the correct style. This does however mean
+that there can be a lot of commit noise to just "fix CI style errors".
+
+2. Have bors apply rustfmt on merge. This would be the painless method, but
+would not act as an example for newcomers.
+
+# Unresolved Questions
+[unresolved]: #unresolved
+
+* Should we do it?
+* If so, which method to use?