Set the `$BUCK_DEBUG_SOY` environment variable before starting Buck
to be able to edit/refresh the `.soy` files in this directory when running the
webserver:

    export BUCK_DEBUG_SOY=1

Add documentation about BUCK_DEBUG_SOY near code that uses it.
Add documentation about BUCK_DEBUG_SOY near code that uses it.

Test Plan: Read documentation; found it useful.
