debugging: add a slide about smem tool which generates memory usage

[clementleger]

https://www.selenic.com/smem/

[tpetazzoni]

Don't know if it is that important, but smem has not seen any release since 2013, and the last commit in the Mercurial repo is from 2015. Perhaps it's the kind of tool that doesn't really need any maintenance: it just works and does its job. But I thought it was worth pointing that out.