Add configuration macros doc page

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

doc/qbk/03_core/8_conf_macros.qbk

@@ -0,0 +1,46 @@
+[/
+    Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail dot com)
+
+    Distributed under the Boost Software License, Version 1.0. (See accompanying
+    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+    Official repository: https://github.com/boostorg/beast
+]
+
+[section Configuration preprocessor definitions]
+
+A number of configuration preprocessor definitions can be used to change
+the behavior of Beast.
+The user should assume that they introduce significant changes to the public part
+of this library's API and make sure that all translation units (usually files)
+compiled and linked into a program use the same combination of configuration macros,
+failure to do so may result in violations of ODR (One Definition Rule).
+
+[table Special Fields
+[[Definition][Description]]
+[
+    [
+        BOOST_BEAST_USE_STD_STRING_VIEW
+    ][
+        Causes Beast to use std::string_view instead of boost::string_view.
+        Requires C++17.
+    ]
+]
+[
+    [
+        BOOST_BEAST_SEPARATE_COMPILATION
+    ][
+        Enables the split compilation mode, which allows the user to compile
+        definitions of non-template entities in a single translation unit, thus
+        improving compilation speed. That translation unit has to include
+        boost/beast/src.hpp in order to compile the definitions.
+    ]
+]
+[
+    [
+        BOOST_BEAST_ALLOW_DEPRECATED
+    ][
+        Enables the use of deprecated APIs within Beast.
+    ]
+]
+]

doc/qbk/03_core/_core.qbk

@@ -96,5 +96,5 @@ effect:
 [include 5_buffers.qbk]
 [include 6_files.qbk]
 [include 7_composed.qbk]
-
+[8_conf_macros.qbk]
 [endsect]