docs: #2255 Add explanation into README.md for Queue-load-leveling pattern (#2707)

* fix: #2255
modify README.md file under queue-load-leveling.
- add explanation section
- add plai-word and wiki say
- add programmatic example with code of taskGenerator and ServiceExecutor classes
- add console output by running the code at local

* ref: add '## explanation' line above real-world example