docs: describe the reasoning a bit why there are 2 timings and why they differ

[staabm]

It would be great if you could describe in the readme with 2-3 sentences why there is this difference between the 2 timings one sees for each resource, so people get the point more clearly what those numbers are about.

a few examples of why those differences exist would be awesome.

[Srikanth-AD]

"It took 3450ms before the user saw this image." The second value in brackets is the time it took the browser to load that specific image.
copied from the description here
https://chrome.google.com/webstore/detail/perfmap/hgpnhiajcdppfbogcpfdgcceepgkhdmk?hl=en&gl=GB