Update README.md (#16437)

* Update README.md

-png, --sample_index options are not available in google-perftools (2.4-0ubuntu5.16.04.1). 
Also, since Ubuntu 16.04 wrongly recommends to install pprof from 'tau' package
"
The program 'pprof' is currently not installed. You can install it by typing:
sudo apt install tau
"
the typical user command should probably be 

google-pprof --pdf --nodecount=100 <filename>

* Add `google-perftools` installation Note.