KTX-Software submodule requires git-lfs - mention this in developer docs

[kring]

As mentioned here in #719 (comment), it seems the KTX-Software submodule can't be properly cloned without git-lfs support. We should mention this requirement in the developer setup docs. It's also worth considering if the lfs files are actually needed for the way we use the library, and if there's a way to prevent the whole thing from breaking when git-lfs is not installed.

[sparkyspider]

I see this this has been marked as #GoodFirstIssue. Can I take this on?

Also, once the build has been completed, there are no further instructions on how to manually install the plugin. Happy to include these aswell. I've posed a question in the Cesium Community Forums: Unreal Plugin.

Clonce resolved, could I add this to the documentation to make it easier for other mac users?

[kring]

Thanks @sparkyspider, we'd be thrilled to see you work on this! I've tried to answer your questions in the community forum (https://community.cesium.com/t/help-installing-on-mac/18614/4).