Add description of metrics in power section to README

**Is your feature request related to a problem? Please describe.**
The metrics shown in the Temparature, Frequency and Utilization are quite self-explanatory. They show the core number (eg. Core 0 for the core number 0) or device name (eg. Iwlwifi_1 for wifi).

But the metrics shown in Power section are difficult to understand for a beginner.
Eg. `package-0,0`, `core,0`, `uncore,0`, `psys,0` etc.

An explanation of these in README will greatly help newcomers.

**Describe the solution you'd like**
If these are standard metrics for all system, a description of these in README would help.
If these metrics can change for various systems, an instruction on where users can find details about their system metrics would help.