contributing: snakeviz replaces pyflame+flamegraph

Replace the instructions for pyflame+flamegraph with simpler ones for snakeviz. For our general use-case this seems to be easier and better. Usage of this tool was demonstrated at the 2019 BuildStream Gathering in January by Daniel Silverstone, when presenting the aggregate results of profiling on many target environments.

Here is the relevant mailing list thread:

"Profiling before the gathering" https://mail.gnome.org/archives/buildstream-list/2019-January/msg00057.html 1b5a49d2