document endianness macros

Author: grisumbras

doc/qbk/01_overview.qbk

@@ -91,6 +91,13 @@ building the library or including the function definitions:
 #include <boost/json/src.hpp>
 ```
 
+[heading Endianness]
+
+Boost.JSON supports both little endian and big endian platforms. It attempts to
+deduce the endianness of the target plarform, but when it can't, you still can
+choose the proper endianness by defining either `BOOST_JSON_LITTLE_ENDIAN` or
+`BOOST_JSON_BIG_ENDIAN`.
+
 [endsect]
 
 [/-----------------------------------------------------------------------------]