[Documentation Request] Create instructions for building Linux AppImages

[scribblemaniac]

We have instructions for building on Linux in docs/build_linux.md that still work as far as I know, however we need to add a section for building AppImages, which has some tricky steps. Additionally, the process is more complex for non-x86_64 architectures, as some of the tools have to be built from source.

[probonopd]

(fyi) Open Build Service has the tools for 32-bit and 64-bit, Intel and ARM.

[J5lx]

We’ll have to see whether maintaining a setup for yet another service (in addition to AppVeyor and Travis) is worth the effort for a small project like ours, but from a quick look some of the features seem really interesting for builds used by end-users (dependencies staying the same and such). Thanks for the tip!