docs: zephyr: add "Installing Requirements and Dependencies"

This patch documents requirements and dependencies necessary to build
mcuboot on Zephyr.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>
diff --git a/docs/readme-zephyr.md b/docs/readme-zephyr.md
index b154edd..fd07244 100644
--- a/docs/readme-zephyr.md
+++ b/docs/readme-zephyr.md
@@ -27,6 +27,15 @@
 `boards/arm/frdm_k64f/frdm_k64f.dts`. Make sure the labels in your board's
 `.dts` file match the ones used there.
 
+## Installing Requirements and Dependencies
+
+Install additional packages required for development with mcuboot:
+
+```
+  cd ~/mcuboot  # or to your directory where mcuboot is cloned
+  pip3 install --user -r scripts/requirements.txt
+```
+
 ## Building the bootloader itself
 
 The bootloader is an ordinary Zephyr application, at least from