README: update with helpful developer information

Provide some clues to people who might want to try out SDK
customizations.

Signed-off-by: Peter A. Bigot <[email protected]>

Author: pabigot

README.md

@@ -1,9 +1,9 @@
 # Zephyr SDK with Crosstool NG
 
-This project is to replace the current Yocto based SDK with a new framework for
-building the Zephyr SDK using crosstool-ng.
+This project is to replace the previous Yocto based SDK with a new framework for
+building the Zephyr SDK using [crosstool-ng](https://crosstool-ng.github.io/).
 
-The repoistory consists of configurations for the various architectures (not
+The repository consists of configurations for the various architectures (not
 fully tests) and a script that builds on Linux (x86_64) and Mac in a consistent
 way, apply patches where needed.
 
@@ -23,6 +23,16 @@ To build for any of the above, run:
 ./go.sh <arch>
 ```
 
+Production builds are done using the script in `.shippable.yml` which
+identifies some packages you'll need to have installed for the above
+command to work.
+
+When building locally be aware that `./go.sh` is written assuming it's
+invoked in a pristine workspace.  That means if you pull an update of
+this repository `./go.sh` will not update the `crosstool-ng` source and
+rebuild the `bin/ct-ng` utility to match the update configuration.  This
+can produce very confusing results.
+
 ## Released Binaries packages
 
 Pre-built binaries releases of the SDK are made and currently only available for