Finished rough draft of doc.
Added a horizontal line to separate new FAQ item.
Clarified that it's mojit inheritance and not inheritance in general that is not supported, also listed what can't be re-used/extended in Mojito.
Fixed a typo.
Merge pull request #921 from zhouyaoji/docs_faq_extend-reuse_mojits

[doc] Added FAQ discussing how to re-use and extend mojits.