docs: Extend readme, more docs

Adds/updates docs.

Bug: 125, 129

Change-Id: I96b8bc07c3817d6b629e546f74c06d7345b3227e
diff --git a/docs/developer_guide.md b/docs/developer_guide.md
index 014d408..a9ba1d8 100644
--- a/docs/developer_guide.md
+++ b/docs/developer_guide.md
@@ -146,7 +146,7 @@
 the boards and distribute the tests across the devices. More boards = faster
 tests!
 
-![](images/stm32f429i-disc1_connected.jpg)
+![development boards connected via USB](images/stm32f429i-disc1_connected.jpg)
 
 ### 2. Launch Test Server
 To allow Ninja to run tests on an arbitrary number of devices, Ninja will send