update readme - D running instructions; remove LDC recommendations si…

[buggins]

…nce it adds only a few percents of performance

[nuald]

I think it's fair and valuable to keep both DMD and LDC metrics available (and I've done it already in the most recent commit). However, I had to change the source code and the configuration:

• Switched from libevent to libasync (the former doesn't work in x86_64 Windows with the current version of vibe.d, and moreover I think that using libasync is recommended by its developers);
• Removed multithreading (caused crashes in macOS with the exit code 11). Besides crashes, I guess it's not fair, because Go and Rust implementation don't use multithreading, but async execution (I guess we can modify Node.js tests to do the same).

[buggins]

Go implementation eats 120% CPU. Can it be done w/o multithreading? Go allocates threads (workers) by default according to number of CPU cores.