Add coding standard info to README

Coseva has recently decided to adopt PSR coding standard and this information should be available on the README:
- [PSR-1 Basic Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-1-basic-coding-standard.md)
- [PSR-2 Coding Style Guide](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)
