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

README.md

@@ -213,11 +213,44 @@ JsonAdapter<Event> jsonAdapter = moshi.adapter(Event.class);
 Event event = jsonAdapter.fromJson(json);
 ```
 
+### Adapter convenience methods
+
+Moshi provides a number of convenience methods for `JsonAdapter` objects:
+- `nullSafe()`
+- `nonNull()`
+- `lenient()`
+- `failOnUnknown()`
+- `indent()`
+- `serializeNulls()`
+
+These factory methods wrap an existing `JsonAdapter` into additional functionality.
+For example, if you have an adapter that doesn't support nullable values, you can use `nullSafe()` to make it null safe:
+
+```java
+String dateJson = "\"2018-11-26T11:04:19.342668Z\"";
+String nullDateJson = "null";
+
+// RFC 3339 date adapter, doesn't support null by default
+// See also: https://github.com/square/moshi/tree/master/adapters
+JsonAdapter<Date> adapter = new Rfc3339DateJsonAdapter();
+
+Date date = adapter.fromJson(dateJson);
+System.out.println(date); // Mon Nov 26 12:04:19 CET 2018
+
+Date nullDate = adapter.fromJson(nullDateJson);
+// Exception, com.squareup.moshi.JsonDataException: Expected a string but was NULL at path $
+
+Date nullDate = adapter.nullSafe().fromJson(nullDateJson);
+System.out.println(nullDate); // null
+```
+
+In contrast to `nullSafe()` there is `nonNull()` to make an adapter refuse null values. Refer to the Moshi JavaDoc for details on the various methods.
+
 ### Parse JSON Arrays
 
 Say we have a JSON string of this structure:
 
-```java
+```json
 [
   {
     "rank": "4",