Add build instructions to Drone images readme

scripts/drone-images/readme.md

@@ -3,3 +3,13 @@ Wagtail CI base images
 
 This directory contains Dockerfiles for building the base images used by
 Wagtail's continuous integration server.
+
+
+Building
+--------
+
+Run the following commands to build all the images:
+
+    docker build -t wagtail-flake8 flake8
+    docker build -t wagtail-jscs jscs
+    docker build -t wagtail-scss scss