contrib : add TODO for preprocessor directives [no ci]

Author: ggerganov

CONTRIBUTING.md

@@ -23,16 +23,24 @@
 - Vertical alignment makes things more readable and easier to batch edit
 - Clean-up any trailing whitespaces, use 4 spaces for indentation, brackets on the same line, `void * ptr`, `int & a`
 - Use sized integer types in the public API
-- Declare structs with `struct x {}` instead of `typedef struct x {} x`
+- Declare structs with `struct foo {}` instead of `typedef struct foo {} foo`
     - In C++ code omit the `struct` keyword whenever it is not necessary
     > [!NOTE]
     > This guideline is yet to be applied to the `llama.cpp` codebase. New code should follow this guideline.
+- Try to follow the existing patterns in the code (indentation, spaces, etc.). In case of doubt use `clang-format` to format the added code
 - Tensors store data in row-major order. We refer to dimension 0 as columns, 1 as rows, 2 as matrices
 - Matrix multiplication is unconventional: [`C = ggml_mul_mat(ctx, A, B)`](https://github.com/ggerganov/llama.cpp/blob/880e352277fc017df4d5794f0c21c44e1eae2b84/ggml.h#L1058-L1064) means $C^T = A B^T \Leftrightarrow C = B A^T.$
-- Try to follow the existing patterns in the code (indentation, spaces, etc.). In case of doubt use `clang-format` to format the added code
 
 ![matmul](media/matmul.png)
 
+- Preprocessor directives
+    - (TODO: add guidelines with examples and apply them to the codebase)
+
+    ```cpp
+    #ifdef FOO
+    #endif // FOO
+    ```
+
 # Naming guidelines
 
 - Use `snake_case` for function, variable and type names