Add README

README.md

@@ -0,0 +1,35 @@
+# Falling sand on the STM32F411CE
+
+## Building
+Run
+```sh
+make build
+```
+
+## Probe for the board
+Run
+```sh
+st-info --probe
+```
+
+## Flashing
+Run
+```sh
+make flash
+```
+
+## Debugging
+
+### `st-info --probe` shows 0KB flash
+```
+> sudo st-info --probe
+Found 1 stlink programmers
+  version:    V2J43S28
+  serial:     0671FF343056363043090732
+  flash:      0 (pagesize: 16384)      <--- 0KB flash
+  sram:       131072
+  chipid:     0x431
+  dev-type:   STM32F411xC_xE
+```
+This can happen when the flash is locked. One way to unlock it is to erase the entire chip via the
+Windows application `ST-Link Util`.