Added missing description of collected metrics

[IzabellaRaulin]

Fixes #14 Missing description of collected metrics plus small fixes in README.md

Summary of changes:

• added missing description of collected metrics
• updated exemplary task manifest in README.md (now it is consistent with the content of /examples/tasks/)
• moved the fragment of "Plugin has ability to read metrics from diskstats..." from Documentation.CollectedMetrics to Documentation, and added the link to kernel doc

  Testing done:

• no needed

[ghost]

LGTM