Dev docs missing build.sh execution permission step

[phillxnet]

The current documentation in the Contributing - Developers Code Build step lacks instruction to enable execution permission on build.sh. This, in an rpm install, is done by the rpm scriptlets; and this execution permission is assumed by rockstor-build.service. The indication of this failure is reported by systemd as follows:

rockstor-build.service: Failed at step EXEC spawning /opt/rockstor/build.sh: Permission denied

And can be worked around by:

chmod u+x /opt/rockstor/build.sh

The above should be added to the indicated Code Build steps to enable subsequent rockstor-build.service function.