[docs] Added llvm/cmake section

[pooja2299]

Summary

Added information about the cmake inside llvm.

[xgupta]

After addressing these changes it seems good to me.

llvm/docs/GettingStarted.rst

824

I think it is in more detail than needed on the getting started page. You may remove OCaml language, Sphinx, TensorFlow compiler, atomic builtins, version control information.

[pooja2299]

Removed redundant lines.

[xgupta]

If @mstorsjo and @jroelofs have no issue I will commit it tomorrow.

[xgupta]

@pooja2299 I think you have made a few good patches, If you want to continue contributing, you may request commit access by following the steps mention here: https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access.

llvm/docs/GettingStarted.rst

 Directory Layout
 ================
 
 One useful source of information about the LLVM source base is the LLVM `doxygen
 <http://www.doxygen.org/>`_ documentation available at
 `<https://llvm.org/doxygen/>`_.  The following is a brief introduction to code
 layout:
 
+``llvm/cmake``
+--------------
+Genereates system build files.
+
+``llvm/cmake/modules``
+  Build configuration for llvm user defined options. Checks compiler version and 
+  linker flags.
+
+``llvm/cmake/platforms``
+  Toolchain configuration for Android NDK, iOS systems and non-Windows hosts to
+  target MSVC.
+
 ``llvm/examples``
 -----------------
 
 - Some simple examples showing how to use LLVM as a compiler for a custom 
   language - including lowering, optimization, and code generation.
 
 - Kaleidoscope Tutorial: Kaleidoscope language tutorial run through the 
   implementation of a nice little compiler for a non-trivial language