[README] Add note on using cmake to perform the build

# The LLVM Compiler Infrastructure

This directory and its subdirectories contain source code for LLVM,

This directory and its sub-directories contain source code for LLVM,

a toolkit for the construction of highly optimized compilers,

optimizers, and runtime environments.

optimizers, and run-time environments.

The README briefly describes how to get started with building LLVM.

For more information on how to contribute to the LLVM project, please

Getting Started](http://clang.llvm.org/get_started.html) page might have more

accurate information.

This is an example workflow and configuration to get and build the LLVM source:

This is an example work-flow and configuration to get and build the LLVM source:

1. Checkout LLVM (including related subprojects like Clang):

1. Checkout LLVM (including related sub-projects like Clang):

     * ``git clone https://github.com/llvm/llvm-project.git``

     * ``cmake -G <generator> [options] ../llvm``

        Some common generators are:

        Some common build system generators are:

        * ``Ninja`` --- for generating [Ninja](https://ninja-build.org)

          build files. Most llvm developers use Ninja.

        Some Common options:

        * ``-DLLVM_ENABLE_PROJECTS='...'`` --- semicolon-separated list of the LLVM

          subprojects you'd like to additionally build. Can include any of: clang,

          sub-projects you'd like to additionally build. Can include any of: clang,

          clang-tools-extra, libcxx, libcxxabi, libunwind, lldb, compiler-rt, lld,

          polly, or debuginfo-tests.

        * ``-DLLVM_ENABLE_ASSERTIONS=On`` --- Compile with assertion checks enabled

          (default is Yes for Debug builds, No for all other build types).

      * Run your build tool of choice!

      * ``cmake --build . [-- [options] <target>]`` or your build system specified above

        directly.

        * The default target (i.e. ``ninja`` or ``make``) will build all of LLVM.

        * The ``check-all`` target (i.e. ``ninja check-all``) will run the

          regression tests to ensure everything is in working order.

        * CMake will generate build targets for each tool and library, and most

        * CMake will generate targets for each tool and library, and most

          LLVM sub-projects generate their own ``check-<project>`` target.

        * Running a serial build will be *slow*.  To improve speed, try running a

          parallel build. That's done by default in Ninja; for ``make``, use

          ``make -j NNN`` (NNN is the number of parallel jobs, use e.g. number of

          CPUs you have.)

        * Running a serial build will be **slow**.  To improve speed, try running a

          parallel build.  That's done by default in Ninja; for ``make``, use the option

          ``-j NNN``, where ``NNN`` is the number of parallel jobs, e.g. the number of

          CPUs you have.

      * For more information see [CMake](https://llvm.org/docs/CMake.html)