Update README.md with more documentation

Author: Andrew Baumhauer

README.md

@@ -309,3 +309,9 @@ feature, which adds `ToSql` and `FromSql` implementations for `chrono`'s
 [BIT and VARBIT](http://www.postgresql.org/docs/9.4/static/datatype-bit.html)
 support is provided optionally by the `bit-vec` feature, which adds `ToSql` and
 `FromSql` implementations for `bit-vec`'s `BitVec` type.
+
+### MACADDR type
+
+[MACADDR](http://www.postgresql.org/docs/9.4/static/datatype-net-types.html#DATATYPE-MACADDR)
+support is provided optionally by the `eui48` feature, which adds `ToSql` and
+`FromSql` implementations for `eui48`'s `MacAddress` type.