documentation: section about related projects

[albertz]

An overview of related projects would be very nice. E.g. GraphTerm, Xterm.js, Hyper, GateOne, hterm, TermKit, Upterm, etc. As the main point for this project is the extended HTML support via escape codes (or that is how I see it), it would be nice to point out how far or what kind of escape codes are supported in other terminal emulators. E.g. iTerm2 has a few extended escape codes.