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