[Release Notes] Add release note for llvm-mca

[andreadb]

Summary

As the title says, this adds a small paragraph about llvm-mca in ReleaseNotes.rst.

Please let me know if it is okay to commit.

Thanks,
Andrea

[Eugene.Zelenko]

docs/ReleaseNotes.rst

48

Please :program: prefix in both references to tool.

[andreadb]

Addressed review comments.

Added :program: prefix to every occurrence of llvm-mca

[hans]

Thanks for adding release notes! Can you include a link to the CommandGuide/llvm-mca.html to make it easier for the curious reader to find more info?

[andreadb]

Hi Hans,

Thanks for the feedback.

I modified the text so that the first occurrence of llvm-mca references the llvm-mca.rst in the CommandGuide.
I have verified that the href in the html page works fine.

[hans]

Great, thanks!

docs/ReleaseNotes.rst

.. NOTE
    functionality, or simply have a lot to talk about), see the `NOTE` below
    for adding a new subsection.
 
 * The LoopInstSimplify pass (-loop-instsimplify) has been removed.
 
 * Symbols starting with ``?`` are no longer mangled by LLVM when using the
   Windows ``x`` or ``w`` IR mangling schemes.
 
+* A new tool named `llvm-mca` has been added. `llvm-mca` is a static performance
+  analysis tool that uses information available in LLVM to statically predict
+  the performance of machine code for a specific CPU.
+
 * Note..
 
 .. NOTE
    If you would like to document a larger change, then you can add a
    subsection about it right here. You can copy the following boilerplate
    and un-indent it (the indentation causes it to be inside this comment).
 
    Special New Feature