Updated README to be clearer on how to develop using the Docker Conta… (#237)

Update README

* Updated README to be clearer on how to develop using the Docker Container Environment vs Local host setup.
* Added on additional optional step for docker container development
* Move SGX and IAS sections to top

Signed-off-by: Ann Umberhocker <ann.umberhocker@ibm.com>

Author: annumberhocker