Add .clang-format file and vendor clang-format-hook as Git submodule

Use 'Mozilla' style with slight modifications:

* Break before binary operators, except assignment operators.
* Usually break before braces ('Allman' style).

.clang-format

+---
+BasedOnStyle: Mozilla
+
+# Break before braces, except for one-line statements
+BreakBeforeBraces: Allman
+
+# Do not break before assignment operators
+BreakBeforeBinaryOperators: NonAssignment

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "plugins/vendor/googletest"]
 	path = plugins/vendor/googletest
 	url = https://github.com/google/googletest.git
+[submodule "plugins/vendor/clang-format-hooks"]
+	path = plugins/vendor/clang-format-hooks
+	url = https://github.com/barisione/clang-format-hooks.git

CHANGELOG.md

@@ -5,6 +5,9 @@
 ### Added
 
 * Extended Miller scaling adapter with additional porosity scaling !122
+* Coding style tools and instructions.
+    [`clang-format-hook`](https://github.com/barisione/clang-format-hooks)
+    as Git submodule !203
 
 ### Changed
 

CONTRIBUTING.md

@@ -68,6 +68,41 @@ templates. Provide information on how your code changes and additions solve the
 problem or implement the feature. Make sure that your MR fulfills the the
 criteria for being merged.
 
+#### C++ Coding Style
+
+The DORiE repository contains a `.clang-format` file in the top-level directory specifying the desired style for C++ code in DORiE.
+We ask developers to use this style when writing new code, ideally by instructing their editor and/or [ClangFormat](https://clang.llvm.org/docs/ClangFormat.html) tool to do so.
+Every change to C++ files introduced by Git commits should be formatted according to this style.
+
+DORiE vendors [clang-format-hooks](https://github.com/barisione/clang-format-hooks) to automate formatting and avoid committing unformated changes.
+It contains a Git pre-commit hook which checks the changes right before committing.
+If the changes to not comply to the coding style, the user is shown the suggested style and prompted to apply the changes or abort the commit.
+Follow these instructions for registering the hook:
+
+1. Install `clang-format`.
+
+    On **macOS**:
+
+    ```bash
+    brew install clang-format
+    ```
+
+    On **Ubuntu**:
+
+    ```bash
+    apt install clang-format
+    ```
+
+2. Register the hook. From the top-level DORiE source directory, execute
+
+    ```bash
+    ./plugins/vendor/clang-format-hooks/git-pre-commit-format install
+    ```
+
+When running in trouble, consult the [original installation instructions](https://github.com/barisione/clang-format-hooks#setup).
+
+Happy coding!
+
 ### Releases
 
 Tasks for creating and publishing a release are listed in the `version-rollout`

clang-format-hooks @ ac35e705

+Subproject commit ac35e705ebf20187b7ff92c29d1411f6a4c8d522