Contributing Guidelines / Code Style

[enqueue]

Please add some contributing guidelines for this project, including code style.

Since the current code styles are different between the contributors, and there are patches with many useless changes in the import order, it might be advisable to enforce some rules via a checkstyle configuration. Perhaps Yandex has already such a configuration, else simply start with one of the popular standards, e.g. Sun / Oracle Code Conventions or Google Java Styleguide.

I am volunteering to help bring the current code base in shape once such a script is installed in the CI.