Add licences (CERN-OHL-S-2.0 for hardware, MIT for firmware/docs) and licence notes

README.md

@@ -142,7 +142,9 @@ S8 UART: 9600 baud, 8N1. BME280 I²C: standard address `0x76` (or `0x77` dependi
 ├── bom/                         bill of materials (CSV)
 ├── docs/                        schematic PDF, layer plots, 3D render
 ├── firmware/                    ESPHome configuration
-└── photos/                      photographs of the assembled board
+├── photos/                      photographs of the assembled board
+├── LICENSE                      CERN-OHL-S-2.0 (hardware)
+└── LICENSE.MIT                  MIT (firmware config, BOM, docs)
 ```
 
 ## Rebuilding the board
@@ -156,6 +158,20 @@ S8 UART: 9600 baud, 8N1. BME280 I²C: standard address `0x76` (or `0x77` dependi
 
 ## License
 
-Not chosen yet. Recommended for a published open-hardware module:
-**CERN-OHL-S-2.0** for the hardware (schematic, layout, footprints) and **MIT** for the firmware
-configuration and scripts. A `LICENSE` file must be added before publishing.
+Dual licensed, SPDX identifiers:
+
+| Scope | Licence | File |
+|---|---|---|
+| Hardware: schematic, layout, symbols, footprints, fabrication data | **CERN-OHL-S-2.0** (CERN Open Hardware Licence v2, strongly reciprocal) | [`LICENSE`](LICENSE) |
+| Firmware configuration, BOM, scripts, documentation | **MIT** | [`LICENSE.MIT`](LICENSE.MIT) |
+
+SPDX notices to keep in derived files:
+
+```
+SPDX-License-Identifier: CERN-OHL-S-2.0
+SPDX-FileCopyrightText: 2026 Hannes Gehrmann
+```
+
+Making a board from these files and distributing it is explicitly allowed; if you modify the design
+and convey it, CERN-OHL-S requires you to publish the modified design under the same licence and to
+mark what you changed (sections 3.2 and 3.3).