Added text about id generator. Removed invalid ids from the examples.

[renatoliveira]

I've changed the readme text a bit to use valid Ids, and added a section about using an Id generator class instead of hardcoding them. 👍

[renatoliveira]

Just made an addition: now it is possible to set the parent with a DescribeFieldResult. This way is safer to set relationships between custom objects. I've had an issue a couple of hours ago where I had two custom objects related to each other and the framework couldn't set the relationship correctly because of a typo.

Relationship was like 'Parent__r' and I wrote 'parent__r'. With this, there's no way to get a typo, even if someone changes the field on setup (like when correcting the lowercase "P" in this case).