[flang] Add release notes for LLVM 11 release

[richard.barton.arm]

Summary

Adding an initial Flang release notes file on the LLVM 11 branch as no
placeholder file was present on master when the branch was taken.

[sscalpone]

flang/docs/ReleaseNotes.rst

91

The documentation is in the
flang/documentation/ directory.

The abi documentation, if generated, is in docs.

[hans]

flang/docs/ReleaseNotes.rst

91

I suppose this ReleaseNotes.rst file should also be in flang/documentation/ ?

But it's a bit confusing to not use the docs/ dir like other sub-projects.

[sscalpone]

No objection to first moving documentation to docs. If someone takes this on, there are links that need to be updated, relatively in the project and in for example Gary's call notes.

[sameeranjoshi]

Thanks @richard.barton.arm I just noticed this revision from the flang-dev thread you commented today.
Take a look at the review at https://reviews.llvm.org/D85470

[sameeranjoshi]

In D84864#2187764, @sscalpone wrote:

No objection to first moving documentation to docs. If someone takes this on, there are links that need to be updated, relatively in the project and in for example Gary's call notes.

@sscalpone can you suggest which link do you want to update and what more details you want me to add?

[richard.barton.arm]

Super - thanks @sameeranjoshi for taking this forward. I'll close this patch and we can do the rest of the work on D85470

flang/docs/ReleaseNotes.rst

+========================================
+Flang 11.0.0 (In-Progress) Release Notes
+========================================
+
+.. contents::
+   :local:
+   :depth: 2
+
+.. warning::
+
+   These are in-progress notes for the upcoming LLVM 11.0.0 release.
+   Release notes for previous releases can be found on
+   `the Download Page <https://releases.llvm.org/download.html>`_.
+
+Introduction
+============
+
+This document contains the release notes for the Flang Fortran
+frontend, part of the LLVM Compiler Infrastructure, release 11.0.0. Here we
+describe the status of Flang in some detail, including major
+improvements from the previous release and new feature work. For the
+general LLVM release notes, see `the LLVM
+documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM
+releases may be downloaded from the `LLVM releases web
+site <https://llvm.org/releases/>`_.
+
+Note that if you are reading this file from a Git checkout, this document
+applies to the *next* release, not
+the current one. To see the release notes for a specific release, please
+see the `releases page <https://llvm.org/releases/>`_.
+
+Known Issues
+============
+
+These are issues that couldn't be fixed before the release. See the bug reports for the latest status.
+
+- ...
+
+Introducing Flang
+=================
+
+Flang is LLVM's Fortran front end and is new for the LLVM 11 release.
+
+Flang is still a work in progress for this release and is included for
+experimentation and feedback.
+
+Flang status
+------------
+
+Flang is able to parse a comprehensive subset of the Fortran language
+and check it for correctness. Flang is not yet able to generate LLVM IR for
+the source code and thus is unable to compile a running binary. 
+
+Flang is able to unparse the input source code into a canonical form and emit
+it to allow testing. Flang can also invoke an external Fortran compiler on this
+canonical input.
+
+Flang's parser has comprehensive support for:
+- Fortran 2018
+- OpenMP 4.5
+- OpenACC 3.0
+
+Major missing features
+----------------------
+
+- Flang is not supported on Windows platforms.
+
+Using Flang
+===========
+
+Usage: ``flang hello.f90 -o hello.bin``
+
+Flang will parse the Fortran file ``hello.f90`` then unparse it to a canonical
+Fortran source file. Flang will then invoke an external Fortran compiler to
+compile this source file and link it, placing the resulting executable
+in ``hello.bin``.
+
+To specify the external Fortran compiler, set the ``F18_FC`` environment
+variable to the name of the compiler binary and ensure it is on your ``PATH``.
+The default value for ``F18_FC`` is ``gfortran``.
+
+When invoked with no source input, Flang will wait for input on standard in.
+When invoked in this way, Flang performs the same actions as if called with
+``-fdebug-measure-parse-tree -funparse`` and does not invoke ``F18_FC``.
+
+For a full list of options that Flang supports, run ``flang --help``.
+
+Additional Information
+======================
+
+Flang's documentation is located in the ``flang/docs/`` directory in
+the LLVM monorepo.
+
+If you have any questions or comments about Flang, please feel free to
+contact us via the `mailing
+list <https://lists.llvm.org/mailman/listinfo/flang-dev>`_.