Add Buck and xctool to json compilation database docs

[• LegNeato]

[silvas]

Do you have a more specific link for buck? After a decent amount of searching I didn't find any direct documentation for this.

Otherwise, LGTM.

docs/JSONCompilationDatabase.rst

-  Figuring out whether things have changed is often an IO bound
    on the build system.
 -  Build systems are inherently sequential in the build graph, for
    example due to generated source code. While tools that run
    independently of the build still need the generated source code to
    exist, running tools multiple times over unchanging source does not
    require serialization of the runs according to the build dependency
    graph.
 
-Supported Systems
-=================
+Supported Build Systems
+=======================
 
-Currently `CMake <http://cmake.org>`_ (since 2.8.5) supports generation
+-  `CMake <http://cmake.org>`_ (since 2.8.5) supports generation
    of compilation databases for Unix Makefile builds (Ninja builds in the
    works) with the option ``CMAKE_EXPORT_COMPILE_COMMANDS``.
+-  `xctool <https://github.com/facebook/xctool>`_ supports generation
+   of compilation databases via the ``json-compilation-database``
+   `reporter <https://github.com/facebook/xctool#reporters>`_.
+-  `Buck <https://buckbuild.com/>`_ supports generation
+   of compilation databases via the ``compilation-database`` flavor.
+
+Other Supported Systems
+=======================
 
 For projects on Linux, there is an alternative to intercept compiler
 calls with a tool called `Bear <https://github.com/rizsotto/Bear>`_.
 
 Clang's tooling interface supports reading compilation databases; see
 the :doc:`LibTooling documentation <LibTooling>`. libclang and its
 python bindings also support this (since clang 3.2); see
 `CXCompilationDatabase.h </doxygen/group__COMPILATIONDB.html>`_.