Add documentation about versioning of the project

[ppodgorsek]

It is difficult to follow which version of Spring is used by which version(s) of this project. Adding a section in the documentation will make it easier for users.
The recommendation is to update the release naming to follow the Spring version, the major version number could match the major Spring version number. (e.g. 5.x.x)
Additionally, this will implicitly define which JDK and other dependency versions are used in each version. (compatible with that version of Spring)

[ppodgorsek]

@philwebb + @springtestdbunit, what do you think of the above suggestion?

[ppodgorsek]

Will be done in a maintained repository under ppodgorsek#2