Add section about null safety (#757)
Add section about null safety (#757)

* Add section about null safety

Null safety is an important feature of the Kotlin language.
The fact that Moshi supports wrapping existing adapters in a null safe or non-null variant is important for null safety.
Therefore it makes sense to promote this feature in the readme, as this is one of the first places people should look for support and such features.

* Move factory method section out of Kotlin section

* Fix syntax highlighting

* Remove isLenient() reference

* Rewrite Kotlin example as Java, reformat comments

square/moshi,square/moshi,square/moshi