[libc++] Re-document how to use <filesystem> with various versions of libc++

This documentation was removed when we added <filesystem> to the dylib
in r356518, but it really should have been updated to reflect the new
state of things. Keeping documentation around doesn't hurt and users
will have an easier time migrating.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@356681 91177308-0d34-0410-b5e6-96231b3b80d8

docs/UsingLibcxx.rst

@@ -49,6 +49,16 @@ An example of using ``LD_LIBRARY_PATH``:
   $ export LD_LIBRARY_PATH=<libcxx-install-prefix>/lib
   $ ./a.out # Searches for libc++ along LD_LIBRARY_PATH
 
+Using ``<filesystem>``
+======================
+
+Prior to LLVM 9.0, libc++ provides the implementation of the filesystem library
+in a separate static library. Users of ``<filesystem>`` and ``<experimental/filesystem>``
+are required to link ``-lc++fs``. Prior to libc++ 7.0, users of
+``<experimental/filesystem>`` were required to link libc++experimental.
+
+Starting with LLVM 9.0, support for ``<filesystem>`` is provided in the main
+library and nothing special is required to use ``<filesystem>``.
 
 Using libc++experimental and ``<experimental/...>``
 =====================================================