Implement unit tests

Unittests as a patch, and a replacement README.md file.

That will help with this 14 year old issue: #893294: Add tests →

I will just commit this and see what happens.

there were the following issues:
- EventDispatcherInterface definition;
- no trailing periods in inline comments;
- wrong parametes in $this->assertTrue($this->workflowTransition->isAllowed($this->account, $from_state, $to_state));

I do not understand the parameters of this last test. They are now back to original state.