Add note about the wrk tool

[stefanchrobot]

• I've added a note about the wrk tool,
• I also added the direct link to installation instructions, since it has to be built from source on Linux. wrk will not work on Windows, based on this comment,
• seems that the value 2000 for the timeout parameter is a mistake, since the default unit is seconds and wrk will try to allocate lots of memory to calculate statistics; changed it to 2s even though it's the default (although the default in code is expressed in miliseconds)

[sasa1977]

Thank you for contributing this!