Document UART

Merges: #2

Author: chrysn

Makefile

@@ -32,9 +32,10 @@ USEMODULE += vfs
 USEMODULE += prng_sha256prng
 USEMODULE += tiny_strerror
 FEATURES_REQUIRED += periph_adc
+FEATURES_REQUIRED += periph_gpio
 FEATURES_REQUIRED += periph_i2c
 FEATURES_REQUIRED += periph_spi
-FEATURES_REQUIRED += periph_gpio
+FEATURES_REQUIRED += periph_uart
 
 USEPKG += nimble