[libc] Add a doc describing the current status of libc runtimes build.

[sivachandra]

Summary

A section briefly mentioning the planned future enhancements has also
been included.

libc/docs/index.rst

.. toctree::
     header_generation
     implementation_standard
     api_test
     layering
     mechanics_of_public_api
     redirectors
     source_layout
     strings
+    runtimes_build

libc/docs/runtimes_build.rst

+Building libc using the runtimes build setup
+============================================
+
+The runtimes build of the LLVM toolchain first builds clang and then builds the
+various runtimes (like ``libc++`` and ``compiler-rt``) and LLVM binutils (like
+``llvm-objcopy`` and ``llvm-readelf``) using the freshly built clang. One can
+build libc also as in the same manner. As of this writing, only the ABI agnostic
+parts of the libc are included when built in that manner. This allows interested
+users to continue using their system libc's headers while linking to LLVM libc's
+implementations when they are available. To build libc using the runtimes build
+setup, one needs to include the ``libc`` project in the list of the enabled
+runtimes when configuring the build:
+
+.. code-block:: shell
+
+   $> cmake ../llvm -GNinja -DLLVM_ENABLE_PROJECTS="llvm;clang" \
+      -DLLVM_ENABLE_RUNTIMES=libc
+
+Note that Ninja is used as the generator in the above CMake command. Hence, to
+actually build the libc, one has to build the Ninja target named ``llvmlibc``:
+
+.. code-block:: shell
+
+   $> ninja llvmlibc
+
+If a different generator is used, then the build command should be suitably
+adapted to build the target ``llvmlibc``. Building that target will produce a
+static archive which includes all ABI agnostic functions available in LLVM libc.
+
+Future direction
+----------------
+
+We plan to enhance the runtimes build of LLVM libc to include ABI sensitive
+parts and to also generate the public headers. Likewise, we would like to
+provide an option to build other runtimes like ``libc++`` and ``compiler-rt``
+against LLVM libc.