[llvm] [Offload] Expand documentation with & build guide (PR #149792)

[Joseph Huber via llvm-commits]

================
@@ -6,12 +6,109 @@
 Welcome to Offload's documentation!
 ===================================
 
+Summary
+-------
+
+The Offload subproject aims at providing tooling, runtimes, and APIs that allow
+users to execute code on accelerators or other "co-processors" that may or may
+not match the architecture of their "host". In the long run, all kinds of
+targets are in scope of this effort, including but not limited to: CPUs, GPUs,
+FPGAs, AI/ML accelerators, distributed resources, etc.
+
+For OpenMP offload users, the project is ready and fully usable. The final API
+design is still under development. More content will show up here and on our
+webpage soon. In the meantime, people are encouraged to participate in our
+meetings (see below) and check our `development board
+<https://github.com/orgs/llvm/projects/24/>`_ as well as the discussions on
+`Discourse <https://discourse.llvm.org/tag/offload>`_.
+
 .. toctree::
    :maxdepth: 2
    :caption: Contents:
 
    offload-api
 
+Meetings
+--------
+
+Every second Wednesday, 7:00 - 8:00am PT, starting Jan 24, 2024. Alternates
+with the OpenMP in LLVM meeting. `invite.ics
+<https://drive.google.com/file/d/1AYwKdnM01aV9Gv9k435ArEAhn7PAer7z/view?usp=sharing>`_
+`Meeting Minutes and Agenda
+<https://docs.google.com/document/d/1PAeEshxHCv22JDBCPA9GXGggLp0t7rsnD_jL04MBbzw/edit?usp=sharing>`_.
+
+
+Building
+--------
+
+A minimal Linux build CMake configuration:
+
+.. code-block:: console
+
+  $ cmake llvm -Bbuild \
+      -DCMAKE_BUILD_TYPE=Release \
+      -DLLVM_ENABLE_PROJECTS='clang;clang-tools-extra;lldb;lld' \
+      -DLLVM_ENABLE_RUNTIMES='offload;openmp' \
+      -DLLVM_BINUTILS_INCDIR=/usr/include \
+      -DLLVM_INSTALL_GTEST=ON
+  $ cmake --build build
+
+* ``LLVM_ENABLE_RUNTIMES`` must include ``openmp`` as it is currently a
+  dependency of ``offload`` during the initial transitional phase of the
+  project.
+* ``LLVM_BINUTILS_INCDIR`` ensures that ``LLVMgold.so`` build is enabled which
----------------
jhuber6 wrote:

Why is this required?

https://github.com/llvm/llvm-project/pull/149792