Add .clang-format

Should not be followed for C files, to avoid mucking `blame`s,
but will be applied during the gradual C++ rollout, since that
will likely touch most lines in the codebase anyway.

README.rst

@@ -73,10 +73,14 @@ The RGBDS source code file structure somewhat resembles the following:
        ├── test/
        │   ├── ...
        │   └── run-tests.sh
+       ├── .clang-format
        ├── CMakeLists.txt
        ├── Makefile
        └── README.rst
 
+.. |clang-format| replace:: ``clang-format``
+.. _clang-format: https://clang.llvm.org/docs/ClangFormat.html
+
 - ``.github/`` - files and scripts related to the integration of the RGBDS codebase with
   GitHub.
 
@@ -99,6 +103,8 @@ The RGBDS source code file structure somewhat resembles the following:
 
 - ``test/`` - testing framework used to verify that changes to the code don't break or modify the behavior of RGBDS.
 
+- ``.clang-format`` - code style for automated formatting with |clang-format|_. The C code does not currently follow this style, but all C++ code should.
+
 3. History
 ----------