Add FAQ entry for the 26MHz error

src/SUMMARY.md

@@ -26,4 +26,5 @@
     - [Wokwi](./tooling/simulating/wokwi.md)
     - [QEMU](tooling/simulating/qemu.md)
 - [Troubleshooting](./troubleshooting/index.md)
-  - [`esp-idf-hal` based projects](./troubleshooting/std.md)
+  - [`esp-idf-hal` based projects](./troubleshooting/std.md)
+  - [`espflash`](./troubleshooting/espflash.md)

src/troubleshooting/espflash.md

@@ -0,0 +1,30 @@
+# espflash
+
+## Error when trying to use espflash: `rtc_clk_init: Possibly invalid CONFIG_XTAL_FREQ setting`
+
+This issue is reported by users using an ESP32 module with a 26MHz crystal oscillator. The root
+cause of the issue is that the default bootloader flashed by espflash expects a 40MHz crystal.
+
+### If you are building an `esp-idf-hal` based project
+
+Make sure your `sdkconfig` is set up properly to use a 26MHz crystal. It needs to contain the
+following configuration options:
+
+```
+CONFIG_ESP32_XTAL_FREQ_26=y
+CONFIG_ESP32_XTAL_FREQ=26
+```
+
+You should also prefer using `cargo-espflash` over `espflash`. `cargo-espflash` integrates with your
+project and it will flash the bootloader that is built next to your project instead of the default
+one.
+
+TODO is this enough? How to get `espflash` working?
+
+### If you are building an `esp-hal` based project
+
+Make sure your HAL ([ESP32](https://docs.rs/esp32-hal/latest/esp32_hal/), [ESP32-C2](https://docs.rs/esp32c2-hal/latest/esp32c2_hal/))
+is configured to the correct crystal frequency. To do this, you must disable the default features
+and enable `xtal-26mhz` (besides the other default features).
+
+INCOMPLETE