docs: added clippy & rustfmt to STYLE_GUIDE.md
# Formatting
+
+- Adhere to [Rust Clippy](link:https://github.com/rust-lang/rust-clippy) & [Rustfmt](link:https://github.com/rust-lang/rustfmt) guidelines.
- Indent using tabs.
- Lines should be longer than 100 characters long only in exceptional circumstances and certainly
no longer than 120. For this purpose, tabs are considered 4 characters wide.
- Indent levels should be greater than 5 only in exceptional circumstances and certainly no
greater than 8. If they are greater than 5, then consider using `let` or auxiliary functions in
order to strip out complex inline expressions.
-- Never have spaces on a line prior to a non-whitespace character
+- Never have spaces on a line prior to a non-whitespace character.
- Follow-on lines are only ever a single indent from the original line.
```rust
