Add a guide for contributors to the README.

[berlin-ab]

I was wondering while I was making my pull request:

• how do you prefer contributions?
• how do you run the entire test suite?

[neoremind]

Hi berlin,

For testcases, please run the following commands one by one against the three modules inside project to execute test suite. If all test cases pass, then you can check the index.html file under target folder to see the test coverage rate.

cd fluent-validator && mvn clean package jacoco:report cd fluent-validator-jsr303 && mvn clean package jacoco:report cd fluent-validator-spring && mvn clean package jacoco:report

For the contribution part, I will add a section to make it clear on how to contribute.