README: Document new memory block syntax
README: Document new memory block syntax

Document the new 'memory' block syntax used to specify the memory
accessed by a load or store instruction.

stsquad/risu,vivier/risu,hw-claudio/risu-aarch64,hw-claudio/risu-aarch64,vivier/risu,stsquad/risu,stsquad/risu