Missing implementation descriptions in readme

[e-shreve-ti]

The README.md for the project doesn't describe the following implementations:

• mf4speed
• mf4stack
• m4speed
• m4stack
• mem_opt

The first four are somewhat self-explanatory (I'm assuming 'speed' is optimized for speed and 'stack' for RAM usage), but it would be nice to have references/information on what optimizations have been done.

[mkannwischer]

the last one is a memory optimized C implementation. Feel free to open a PR to the README to clarify.

We commonly document implementations through pull request descriptions. Those usually include links to papers, but I agree that it can be tedious to trace through the long chain of papers. A text file per implementation containing the relevant references would be nice. I won't have time to create that any time soon unfortunately.