Update build-from-source instruction

[lackhole]

Explain what you would like to see improved

Update Building from source section of README.
LLVM has been updated

Optional: share how it could be improved

• nvptx should be NVPTX. Build targets are case-sensitive. See https://llvm.org/docs/CMake.html
• CMAKE_BUILD_TYPE is mandatory.

To Reproduce

Copy and paste current README

First error

CMake Error at CMakeLists.txt:90 (message):
  

  No build type selected.  You need to pass -DCMAKE_BUILD_TYPE=<type> in
  order to configure LLVM.

  Available options are:

    * -DCMAKE_BUILD_TYPE=Release - For an optimized build with no assertions or debug info.
    * -DCMAKE_BUILD_TYPE=Debug - For an unoptimized build with assertions and debug info.
    * -DCMAKE_BUILD_TYPE=RelWithDebInfo - For an optimized build with no assertions but with debug info.
    * -DCMAKE_BUILD_TYPE=MinSizeRel - For a build optimized for size instead of speed.

  Learn more about these options in our documentation at
  https://llvm.org/docs/CMake.html#cmake-build-type

Second error

(After providing CMAKE_BUILD_TYPE)

CMake Error at CMakeLists.txt:898 (message):
  The target `nvpts' is experimental and must be passed via
  LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.

It is hard to resolve this error only with the diagnostic message since the real reason is that target is case-sensitive, not because nvptx is experimental.

Setup

Additional context

Should I open a PR?

[ferdymercury]

Should I open a PR?

Thanks for reporting, yes please, at https://github.com/root-project/root

[gabemorris12]

This is still not fixed, and it's a single line in the readme.

[ferdymercury]

Feel free to transfer this PR (#518) or (#523) to the upstream repo: https://github.com/root-project/root/pulls