Automate formatter and checker

Signed-off-by: Andrea Lamparelli <[email protected]>

.gitignore

@@ -69,7 +69,10 @@ yarn.lock*
 
 react-app-env.d.ts
 
-#docs
+# docs
 docs/site/.hugo_build.lock
 docs/site/public/
 docs/site/resources/
+
+# code format
+.cache

CONTRIBUTING.md

@@ -49,3 +49,40 @@ git remote add upstream https://github.com/Hyperfoil/Horreum.git
 ```
 git checkout -b myfeature master
 ```
+
+### Code Style
+
+Horreum has a strictly enforced code style. Code formatting is done by the Eclipse code formatter, using the config files found in the `config/` directory. By default, when you run `mvn install`, the code will
+be formatted automatically. When submitting a pull request the CI build will fail if running the formatter results in any code changes, so it is recommended that you always run a full Maven build before submitting a pull request.
+
+If you want to run the formatting without doing a full build, you can run `mvn process-sources`.
+
+#### Eclipse Formatting Setup
+
+Open the *Preferences* window, and then navigate to _Java_ -> _Code Style_ -> _Formatter_. Click _Import_ and then
+select the `eclipse-formatting.xml` file in the `config/` directory.
+
+Next navigate to _Java_ -> _Code Style_ -> _Organize Imports_. Click _Import_ and select the `eclipse.importorder` file.
+
+#### IDEA Formatting Setup
+
+Open the _Preferences_ window (or _Settings_ depending on your edition), navigate to _Plugins_ and install
+the [Adapter for Eclipse Code Formatter](https://plugins.jetbrains.com/plugin/6546-eclipse-code-formatter) from the
+Marketplace.
+
+Restart your IDE, open the *Preferences* (or *Settings*) window again and navigate to _Adapter for Eclipse Code
+Formatter_ section on the left pane.
+
+Select _Use Eclipse's Code Formatter_, then change the _Eclipse workspace/project folder or config file_ to point to the
+`eclipse-formatting.xml` file in the `config/` directory. Make sure the _Optimize Imports_ box is
+ticked. Then, select _Import Order from file_ and make it point to the `eclipse.importorder` file in the `config/` 
+directory.
+
+Next, disable wildcard imports:
+navigate to _Editor_ -> _Code Style_ -> _Java_ -> _Imports_
+and set _Class count to use import with '\*'_ to `999`. Do the same with _Names count to use static import with '\*'_.
+
+#### Git Formatting Setup
+
+If you want to be sure that the _formatting_ is properly executed before committing, you can make use of the git hooks, in particular the `pre-commit` one.
+Simply override the `.git/hooks/pre-commit` script with a custom logic that runs `mvn formatter:format impsort:sort` to format the source code and the remember to `git add` all changed files such that they are included in the ongoing commit.