Add boot serial on Mynewt docs
Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/readme-mynewt.md b/docs/readme-mynewt.md
index f1fbcf3..ee88ecb 100644
--- a/docs/readme-mynewt.md
+++ b/docs/readme-mynewt.md
@@ -36,3 +36,17 @@
in this new format. That is done by passing the extra parameter `-2` as in:
`newt create-image <target> <version> <pubkey> -2`
+
+# Boot serial functionality with Mynewt
+
+Building with `BOOT_SERIAL: 1` enables some basic management functionality
+like listing images and uploading a new image to `slot0`. The serial bootloader
+requires that `mtu` is set to a value that is less than or equal to `256`.
+This can be done either by editing `~/.newtmgr.cp.json` and setting the `mtu`
+for the connection profile, or specifying you connection string manually as in:
+
+```
+newtmgr --conntype serial --connstring "dev=/dev/ttyUSB0,mtu=256" image upload -e blinky.img
+```
+
+where `/dev/ttyUSB0` is your serial port.