Add guide to add a new module to Dubbo

[ralf0131]

Create a new page under contributor guide, to document how to add a new module to Dubbo:

1. Update dubbo-all/pom.xml
2. Add your dependency to dubbo-bom/pom.xml
3. External dependencies should go to dubbo-dependency-pom/pom.xml
4. Add your dependency to dubbo-distribution/pom.xml
5. Update dubbo-dependencies if necessary
6. Check the License of your dependencies to make sure they are
   compatible with Apache
7. Check the package name is "org.apache.dubbo".

And explain why each step is necessary.