document endianness macros

Author: grisumbras

doc/qbk/overview.qbk

@@ -101,7 +101,18 @@ building the library or including the function definitions:
 #include <boost/json/src.hpp>
 ```
 
-[heading Supported Compilers]
+[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]
+
+[/-----------------------------------------------------------------------------]
+
+[section Supported Compilers]
 
 Boost.JSON has been tested with the following compilers: