Add missing step to build walkthrough
* Add missing step
Added `make prereqs` before building.
* Update acsengine.md
Corrected spelling.
@@ -14,7 +14,15 @@ This means that you can edit your source code normally in your favorite editor o
machine, while still being able to compile and test inside of the Docker container (the
same environment used in our Continuous Integration system).
-When the execution of `devenv.{ps1,sh}` completes, you should find the console logged into the container. As a final step, in order to get the `acs-engine` tool ready, you should build the sources with:
+When the execution of `devenv.{ps1,sh}` completes, you should find the console logged into the container.
+
+Now we need to do a one-time call to setup the prerequisites.
+
+```
+make prereqs
+```
+
+As a final step, in order to get the `acs-engine` tool ready, you should build the sources with:
```
make build
