Bug#879198: Guide to installing

[Jeronimo Pellegrini <j_p@aleph0.info>]

The following document:

https://gist.github.com/gelim/a840a99d15cb765cb7b65105b72f00c4

is a guide to installing plain ShareLaTeX (without Docker).
Basically, the author shows how to get the source for the
Docker image and make the installation from it on a Ubuntu
system. This would perhaps help a Debian packager.

Note that this installation method is not supported by
the upstream authors of ShareLaTeX (they only support
installation of the Docker image provided by them).

J.