Integrate board photos into README, photos index, and changelog

README.md

@@ -18,6 +18,29 @@ running.
 
 ![3D render](docs/esp-sensair-3d-top.png)
 
+## Photos
+
+Photos of the assembled prototype are in [`photos/`](photos/) and attached to the
+[latest release](https://git.honney333.work/honney/esp_sensair/releases/latest).
+
+| | |
+|---|---|
+| ![Assembled board, front side](photos/frontside.jpg) | ![Assembled board, back side](photos/backside.jpg) |
+| Front side (assembled) | Back side (assembled) |
+
+| | |
+|---|---|
+| ![Front side, soldered](photos/frontside_soldered.jpg) | ![Back side, soldered](photos/backside_soldered.jpg) |
+| Front side, soldered | Back side, soldered |
+
+Running, with an active CO₂ measurement:
+
+![Active CO₂ measurement](photos/active_co2_measurement.jpg)
+
+Live values on the Home Assistant dashboard:
+
+![Home Assistant dashboard](photos/sensor_current_homeassistant.png)
+
 ## Block diagram
 
 ```
@@ -177,7 +200,7 @@ ESPHome on `esp32-c3-devkitm-1`, framework **esp-idf**. Config template:
 ├── bom/                         bill of materials (CSV)
 ├── docs/                        schematic PDF, layer plots, 3D render
 ├── firmware/                    ESPHome config templates, secrets templates, notes
-├── photos/                      photographs of the assembled board
+├── photos/                      photographs of the assembled board (also release assets)
 ├── CHANGELOG.md                 release history
 ├── LICENSE                      CERN-OHL-S-2.0 (hardware)
 └── LICENSE.MIT                  MIT (firmware config, BOM, docs)