Add warning to readme about DevTools performance glitch in Chrome

[Daninet]

#46

https://bugs.chromium.org/p/chromium/issues/detail?id=1226418#c14

[alexanderbkl]

Same thing haha, been struggling when I had Chrome developer console open and didn't notice it was the console at first. The digestion took like 10 times more to process.
I'm using sha256 hasher to update in a FileReader. Other than that, no problems at all.