Add install script to releases and remove .gitkeep from prod

[SolsticeSpectrum]

Context

Currently the way to install gelly is to manually extract the zip into Garry's Mod folder but removing it is a bit of a pain

Scope

This provides an easy to use installer with uninstall option in place

Acceptance Criteria

Ideally it should put this into release next to the zip Also maybe stop including .gitkeep files, they aren't a big issue but there is no reason to have them there

Other

Here is the install script: gelly_install.txt