Add a documentation of the counters feature implementation in
docs/dev/counters.md.
The documentation is taken from the wiki and updated according to the
current state of the code - legacy details are removed, and a section
about the counter id is added.
This folder contains developer-oriented documentation
concerning the ScyllaDB codebase.
We also have a wiki, which contains
additional developer-oriented documentation. There is currently no clear
definition of what goes where, so when looking for something be sure to check
both.