Create README in hardware folder

hardware/README.md

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+### Cores have been moved
+Originally, the main Arduino repository contained Arduino
+platforms/cores in this directory. Since then, these have been moved to
+their own repositories instead (see commit 2e98854 and d6b40d0).
+
+The official Arduino cores can now be found here:
+ - https://github.com/arduino/ArduinoCore-avr
+ - https://github.com/arduino/ArduinoCore-sam
+ - https://github.com/arduino/ArduinoCore-samd
+
+##### Migrating commits
+If you need to move commits and/or pull requests over from this
+repository into the separate core repositories, an approach to handle
+that mostly automatically is suggested below. These are for the avr
+core, but with some changes, it should apply to sam as well.
+
+	# Start out in the Arduino repo, by rebasing your branch on top of the
+	# last commit that still contained the avr core. This makes sure that
+	# any conflicts are resolved before transferring the commits, since git
+	# am is not so helpful with conflicts.
+	git checkout my-branch
+	git rebase -i 950d88dcbe7b9b2d348fb25b5ffcd0c6d2d30b97
+
+	# Then, generate patch files for all of your commits, into the patches
+	# directory.
+	git format-patch -o patches 950d88dcbe7b9b2d348fb25b5ffcd0c6d2d30b97
+
+	# These steps are optional, but if your commits contain changes to other
+	# files than the avr core, this re-applies your commits with only the
+	# avr core changes, and regenerates the patches
+	git checkout -b tmp-branch 950d88dcbe7b9b2d348fb25b5ffcd0c6d2d30b97
+	git am --include 'hardware/arduino/avr/*' patches/*
+	rm -rf patches/
+	git format-patch -o patches 950d88dcbe7b9b2d348fb25b5ffcd0c6d2d30b97
+
+	# Then, in the ArduinoCore-avr repo, create a new branch on the commit
+	# matching the Arduino repo.
+	git checkout -b my-branch b7c607663fecc232e598f2c0acf419ceb0b7078c
+
+	# Apply our previously generated patches (update the path to point to
+	# wherever you generated the patches previously). -p4 tells git am to
+	# strip the a/hardware/arduino/avr part of the paths before applying.
+	git am -p4  /path/to/patches/*
+
+	# Finally, rebase on top of master
+	git rebase origin/master