Added README and LICENSE documents

LICENSE

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+The MIT License (MIT)
+
+Copyright (c) 2021 Thomas Kolb (cfr34k)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE. 

README.md

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+# Firmware for the LNSC-2420 Solar Charger
+
+The LNSC-2420 is a PWM-based solar charge regulator for 24 V systems supporting
+up to 20 A charging current.
+
+Noteworthy features:
+
+- High-side switching with N-Channel MOSFETs:
+  - Gate voltage is provided by a charge pump which doubles the battery
+    voltage.
+  - The MOSFETs are controlled using GPIOs.
+- Current and voltage measurement:
+  - The solar current, load current, solar voltage, battery voltage and charge
+    pump output voltage are measured using the internal ADC of the STM32F030.
+- Low power usage:
+  - The charge pump is disabled if both power FETs are off.
+  - Additionally, the microcontroller is put into deep sleep (Stop Mode) if no
+    solar power is available and the battery voltage is in undervoltage
+    protection.
+- LED status display:
+  - 6 LEDs show the internal charging and discharging state.
+  - The LEDs are controlled using Charlieplexing (i.e. only 3 GPIOs are needed)
+- Protections:
+  - Solar panel reverse current
+  - Battery overvoltage
+  - Battery undervoltage
+  - Load current limit
+
+## Calibration
+
+The sensing hardware should be quite accurate, but some minor error always
+remains. They can be compensated via the parameters in `src/calibration.h`.
+
+To adjust those parameters, first set all `CAL_FACTOR`s to 1000, recompile and
+flash the firmware.  Then watch the RS485 output, which contains all
+measurements in a line starting with `MEAS`. Apply a known current/voltage for
+each measurement and calculate:
+
+                              Actual value
+    CAL_FACTOR = 1000 * -----------------------
+                        Value shown by firmware
+
+Round to the nearest integer and replace the 1000 in `src/calibration.h` by the
+calculated value.
+
+## Customization
+
+All customizable parameters can be adjusted in `src/config.h`.
+
+## License
+
+The code in this repository (except submodules) is licensed under the [MIT license](LICENSE).