Update README information

[dgerod]

• Examples are outdated, see The rosplan_pytools.init() method cannot be found #19
• Explain that OrderDict must be used with current ROSPlan in case of Python 2, see issue in ROSPlan.
• Postconditions, see 'CheckActionAndProcessEffects' uses two different ways to call KB of ROSPlan, they should be unified #18