init adding of formatting instructions

Author: AoifeHughes

developers/contributing/index.qmd

@@ -49,6 +49,28 @@ julia --project=. -e 'import Pkg; Pkg.test(; test_args=ARGS)' -- optim hmc --ski
 
 Or otherwise, set the global `ARGS` variable, and call `include("test/runtests.jl")`. 
 
+### Code Formatting
+
+Turing uses [JuliaFormatter.jl](https://github.com/domluna/JuliaFormatter.jl) to ensure consistent code style across the codebase. **All code must be formatted before submitting a pull request**, and ideally with every commit.
+
+#### Installing JuliaFormatter
+
+We use version 1 of JuliaFormatter. Install it in your **global Julia environment** (not the project environment, as adding it to the Project.toml would make it invalid):
+
+```julia
+julia -e 'using Pkg; Pkg.activate(); Pkg.add(name="JuliaFormatter", version="1"); Pkg.pin("JuliaFormatter")'
+```
+
+#### Formatting Code
+
+To format all Julia files in the current directory and subdirectories:
+
+```julia
+julia -e 'using JuliaFormatter; format(".")'
+```
+
+Run this command from the root of the repository before committing your changes. This ensures your code follows the project's formatting standards and maintains consistency across the codebase.
+
 ### Style Guide
 
 Turing has a style guide, described below.