docs(ProcessInstanceAssert): document new assertions 'isWaitingFor', 'isNotWaitingFor'

Affects #29
feat(ProcessInstanceAssert): create new assertion 'isWaitingFor' to verify process instances

Affects #29
feat(ProcessInstanceAssert): create new assertion 'isNotWaitingFor' to verify process instances

Closes #29
fix(infrastructure): allow to use non-default process engine in case it is the only one registered with ProcessEngines

Closes #32
docs(readme): fix a few typos and add a few hints to feature versions