docs: add Zig to External Open Source Projects

[andrewrk]

Summary

Zig has an llvm6 branch with all tests passing. It's planned to release Zig 0.2.0 directly after LLVM 6.0.0 is released.

[delcypher]

@andrewrk Could you rebase? I just landed a change (r322287) that likely conflicts with this.

[andrewrk]

rebase

[delcypher]

@andrewrk LLVM 4.0 had projects listed alphabetically but it looks like LLVM 5.0 did not so I'm not sure which precedent to follow. @hans how do you want projects ordered?

[hans]

Sorry for the slow reply; back from vacation now.

lgtm

Alphabetical order would be preferred, but I can fix that afterwards if you like.

(Go ahead and commit directly to the branch, of let me know if you'd like me to do it.)

[andrewrk]

Hans, thank you for reviewing. I do not have commit access; would you mind committing it for me, in whatever order you prefer?

[hans]

In D41875#977056, @andrewrk wrote:

Hans, thank you for reviewing. I do not have commit access; would you mind committing it for me, in whatever order you prefer?

Committed r322567. Thanks!

llvm/branches/release_60/docs/ReleaseNotes.rst

 It is built on top of LLVM, Clang, LibFuzzer, and Z3.
 
 The solver works by generating a C++ program where the reachability of an
 `abort()` statement is equivalent to finding a satisfying assignment to the
 constraints. This program is then compiled by Clang with `SanitizerCoverage
 <https://releases.llvm.org/6.0.0/tools/clang/docs/SanitizerCoverage.html>`_
 instrumentation and then fuzzed using :doc:`LibFuzzer <LibFuzzer>`.
 
+Zig Programming Language
+------------------------
+
+`Zig <http://ziglang.org>`_  is an open-source programming language designed
+for robustness, optimality, and clarity. It is intended to replace C. It
+provides high level features such as Generics,
+Compile Time Function Execution, and Partial Evaluation, yet exposes low level
+LLVM IR features such as Aliases. Zig uses Clang to provide automatic
+import of .h symbols - even inline functions and macros. Zig uses LLD combined
+with lazily building compiler-rt to provide out-of-the-box cross-compiling for
+all supported targets.
+
 
 Additional Information
 ======================
 
 A wide variety of additional information is available on the `LLVM web page
 <http://llvm.org/>`_, in particular in the `documentation
 <http://llvm.org/docs/>`_ section.  The web page also contains versions of the
 API documentation which is up-to-date with the Subversion version of the source
 code.  You can access versions of these documents specific to this release by
 going into the ``llvm/docs/`` directory in the LLVM tree.
 
 If you have any questions or comments about LLVM, please feel free to contact
 us via the `mailing lists <http://llvm.org/docs/#maillist>`_.