misc: Added RELEASE-NOTES.md

These release notes will enable users to keep track of the main changes
between gem5 versions. The RELEASE-NOTES.md should also notify the user
of any API changes they should be aware of when using this release of
gem5.

Notes on major changes should be apended to the end of the list as
they are added to the repo.

Change-Id: If75748306a8086b2abbe2f912dbb2ec9f1a6895f
Issue-on: https://gem5.atlassian.net/browse/GEM5-439
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27807
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

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+# Version 20.0.0.0
+
+* Compiling and running gem5 with Python 3 is now fully supported.
+* Compiling and running gem5 with GCC 8 and 9 is now supported.
+* Scons-based tests have been migrated to the testlib framework. Please consult TESTING.md for more information on how these may be run.
+* Support for the ALPHA ISA has been dropped.
+* Memory SimObjects can now be initialized using an image file using the image_file parameter.
+* The m5 utility has been revamped with a new build system based on scons, tests, and updated and more consistent feature support.
+* Robust support for marshalling data from a function call inside the simulation to a function within gem5 using a predefined set of rules.
+* Workload configuration pulled out into its own object, simplifying the System object and making workload configuration more modular and flexible.