doc: draft v2.1 release notes

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/release_notes/release_notes_2.1.rst

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+.. _release_notes_2.1:
+
+ACRN v2.1 (August 2020)
+#######################
+
+We are pleased to announce the release of the Project ACRN
+hypervisor version 2.1.
+
+ACRN is a flexible, lightweight reference hypervisor that is built with
+real-time and safety-criticality in mind. It is optimized to streamline
+embedded development through an open source platform. Check out
+:ref:`introduction` introduction for more information.  All project ACRN
+source code is maintained in the
+https://github.com/projectacrn/acrn-hypervisor repository and includes
+folders for the ACRN hypervisor, the ACRN device model, tools, and
+documentation. You can either download this source code as a zip or
+tar.gz file (see the `ACRN v2.1 GitHub release page
+<https://github.com/projectacrn/acrn-hypervisor/releases/tag/v2.1>`_) or
+use Git clone and checkout commands::
+
+   git clone https://github.com/projectacrn/acrn-hypervisor
+   cd acrn-hypervisor
+   git checkout v2.1
+
+The project's online technical documentation is also tagged to
+correspond with a specific release: generated v2.1 documents can be
+found at https://projectacrn.github.io/2.1/.  Documentation for the
+latest under-development branch is found at
+https://projectacrn.github.io/latest/.
+
+ACRN v2.1 requires Ubuntu 18.04.  Follow the instructions in the
+:ref:`rt_industry_ubuntu_setup` to get started with ACRN.
+
+We recommend that all developers upgrade to ACRN release v2.1.
+
+What's new in v2.1
+******************
+
+* Preempt-RT Linux has been validated as a pre-launched realtime VM.
+
+* A Trusted Platform Module (TPM) MMIO device can be passthroughed to a
+  pre-launched VM (with some limitations discussed in
+  :ref:`mmio-device-passthrough`).  Previously passthrough was only
+  supported for PCI devices.
+
+* Open Virtual Machine Firmware (OVMF) now uses a Local Advanced
+  Programmable Interrupt Controller (LAPIC) timer as its local time
+  instead of the High Precision Event Timer (HPET).
+
+
+Improvements, updates, and corrections have been made throughout our documentation,
+including these:
+
+* :ref:`contribute_guidelines`
+* :ref:`hv_rdt`
+* :ref:`ivshmem-hld`
+* :ref:`mmio-device-passthrough`
+* :ref:`virtio-net`
+* :ref:`getting-started-building`
+* :ref:`acrn_configuration_tool`
+* :ref:`pre_launched_rt`
+* :ref:`rdt_configuration`
+* :ref:`using_hybrid_mode_on_nuc`
+* :ref:`using_partition_mode_on_nuc`
+* :ref:`using_windows_as_uos`
+* :ref:`debian_packaging`
+
+Fixed Issues Details
+********************
+
+.. comment Issues should be carefully formatted like this
+   - :acrn-issue:`3715` -  Add support for multiple RDT resource allocation and fix L3 CAT config overwrite by L2
+
+Known Issues
+************