This is an executable Linux-only "updater" that installs a patched version of libxml2 into an existing Sigil installation. It addresses a bug in libxml2-2.9.3 and higher that affects how QtWebKit renders html entities.
Type a visible html entity into Sigil's Code View. If it appear twice in Sigil's Preview or Book View, then you have the buggy version of libxml2.
Download the installer (make sure its executable bit is set) and use sudo ./sigil_libxml2_update.run to install.
You'll have the option to bail out if you get cold feet.
See the README for more options/explanations.