Missing doc/explanation for the nightly WPE runner

[philn]

In this post I briefly mentioned a script that can be used to download/run WPE/WKGTK nightlies from the buildbots: https://base-art.net/Articles/webkitgtk-and-wpe-now-supporting-videos-in-the-img-tag/

$ wget https://raw.githubusercontent.com/WebKit/webkit/master/Tools/Scripts/webkit-flatpak-run-nightly
$ chmod +x webkit-flatpak-run-nightly
$ python3 webkit-flatpak-run-nightly MiniBrowser https://colinbendell.github.io/webperf/img-mp4/

This script locally installs our new Flatpak-based developer SDK in ~/.cache/wk-nightly and then downloads a zip archive of the build artefacts from servers recently brought up by my colleague Carlos Alberto Lopez Perez, many thanks to him :). The downloaded zip file is unpacked in /tmp and kept around in case you want to run this again without re-downloading the build archive. Flatpak is then used to run the binaries inside a sandbox! This is a nice way to run the bleeding edge of the web-engine, without having to build it or install any distro package.

CC @bkardell

[philn]

Actually I rediscovered a page I wrote some months ago: https://wpewebkit.org/about/flatpak.html
@bkardell is anything missing there?