o Update to clarify when to use docker vs. source installation. 

o Give pointer to docker setup instructions

Updates for ubuntu20

See merge request !12

Also updated readme to reflect change

Many of the README details and build scripts assumed environment
variables that may not exist in the open source release.
This commit cleans up much of it, and reduces the README to
what's needed.

Update zax_driver.cpp with corrected hammock ranges.

See merge request allnp/zafl_umbrella!10