docs: add a style guide

jj-vcs#5685

Author: martinvonz

docs/contributing.md

@@ -115,14 +115,17 @@ directory](https://github.com/jj-vcs/jj/tree/main/cli/src/commands). Working
 on them requires setting up a Rust development environment, as described
 below, and may occasionally require adjusting a test.
 
-
 ## Learning Rust
 
 In addition to the [Rust Book](https://doc.rust-lang.org/book/) and the other
 excellent resources at <https://www.rust-lang.org/learn>, we recommend the
 ["Comprehensive Rust" mini-course](https://google.github.io/comprehensive-rust/)
 for an overview, especially if you are familiar with C++.
 
+## Style guide
+
+See [here](style_guide.md).
+
 ## Setting up a development environment
 
 To develop `jj`, the mandatory steps are simply

docs/style_guide.md

@@ -0,0 +1,12 @@
+# Style guide
+
+## Panics
+
+Panics are not allowed, especially in code that may run on a server. Calling
+`.unwrap()` is okay if it's guaranteed to be safe by previous checks or
+documented invariants. For example, if a function is documented as requiring
+a non-empty slice as input, it's fine to call `slice[0]` and panic.
+
+## Markdown
+
+Try to wrap at 80 columns. We don't have a formatter yet.