Pico Based Systems lesson needs updating

[0joshuaolson1]

From @b1conrad on October 26, 2017 20:43

1. there is now no event:attr("new_child")
2. rulesets cannot be registered programmatically (only installed)
3. clarify and/or add more explanation about two rules being needed for the same event

Copied from original issue: Picolab/pico-engine#337

[0joshuaolson1]

@b1conrad Why 2.?

https://picolabs.atlassian.net/wiki/spaces/docs/pages/32947818/engine#engine-registerRuleset