message: Mention clang-format in the developer guide
diff: @@ -15,6 +15,14 @@ LLVM style guide:
  argument are passed by non-const reference in general.
  * IR constructs are not designed for [const correctness](UsageOfConst.md).
  +Please run clang-format on the files you modified with the `.clang-format`
  +configuration file available in the root directory. Check the clang-format
  +[documentation](https://clang.llvm.org/docs/ClangFormat.html) for more details
  +on integrating it with your development environment. In particular, if clang is
  +installed system-wide, running `git clang-format origin/master` will update the
  +files in the working directory with the relevant formatting changes; don't
  +forget to include those to the commit.
  +
  ## Pass name and other command line options
  To avoid collision between options provided by different dialects, the naming
  