Add README to the repository

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

README.md

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+Nitro FPGA build system
+=======================
+
+This repository contains helpers to build FPGA bitstream using the open-source
+toolchain and make use of re-usable cores, automatically track dependencies
+between them etc ...
+
+Using it is by no means a requirement to re-use any of the "Nitro" FPGA cores,
+it's just a convenience.
+
+For example on how to use this build system, look at the `Makefile` and
+`no2core.mk` in the FPGA cores repositories and in the projects of the
+`smunaut/ice40-playground` repository.
+
+
+Limitations
+-----------
+
+The current version was just extracted from the `ice40-playground` repository
+and as such only supports the ice40 architecture so far. I have a modified
+version for ECP5 that was used in the Hack-a-day badge repository but I have
+yet to merge the two.
+
+
+License
+-------
+
+All the files in this repository are licensed under the terms of the
+BSD 3-Clause "New" or "Revised" License.
+
+See LICENSE file for full text.