Created clang-format (markdown)

clang-format.md

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+New code should be formatted according to .clang-format file before committing it in a pull request. ( https://clang.llvm.org/docs/ClangFormat.html )
+
+It can be installed with `apt install clang-format` or `zypper in clang-tools`
+
+
+## Checking XS files
+
+To also correct the formatting of XS files, configure clang-format to use the clang-format-xs wrapper like so
+```
+git config clangformat.extensions c,h,xs
+git config clangformat.binary $PWD/.github/workflows/clangformat/clang-format-xs
+```
+
+## Manual check after `git add`
+
+Use:
+```
+git clang-format
+```
+
+
+## Auto-check format on commit
+
+To ensure the clang-format formatting before committing, I found this tool:
+
+https://github.com/ailin-nemui/clang-format-hooks
+
+To use it, first clone the repo ( `git clone https://github.com/ailin-nemui/clang-format-hooks.git` )
+
+Then, run the installer script inside your Irssi clone:
+
+```
+cd /path/to/src/irssi
+../clang-format-hooks/git-pre-commit-format install
+```
+
+The next time you try to commit, the formatting will be checked!
+
+
+## Emacs integration
+
+See [Emacs](Emacs)