diff --git a/Makefile b/Makefile
index 167957d..746752d 100644
--- a/Makefile
+++ b/Makefile
@@ -24,7 +24,7 @@ TARGET = bluepill-i2c-gw
 # debug build?
 DEBUG = 1
 # optimization
-OPT = -Og
+OPT = -Os
 
 
 #######################################
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..e5c2753
--- /dev/null
+++ b/README.md
@@ -0,0 +1,32 @@
+# Bluepill USB-I2C gateway
+
+## Hardware
+
+This is a firmware for STM32F103R8T6 Bluepill.
+
+## Customization
+
+The project can be customized using STM32CubeMX.
+
+there's some changes in the i2c HAL driver, careful!
+
+line 6364
+
+```
+  // HACK - ignore HAL_I2C_ERROR_AF
+  if (hi2c->ErrorCode != HAL_I2C_ERROR_NONE && hi2c->ErrorCode != HAL_I2C_ERROR_AF)
+```
+
+this allows receiving frames of unknown length
+
+## Building & programming
+
+use the makefile and flash using a st-link dongle.
+
+Requires: Make, arm-none-eabi-gcc, st-flash
+
+```
+make -j
+make flash
+```
+