Omnibus PR with fixes for #333, #334, #335, and README updates

[nukeandbeans]

This PR fixes issues introduced with PR #333. It fixes #334 and closes #335. All changes that were not present in the release 7521cd7d1c2cc3b9d097d702567aedbd74b2c387 and were in #333 are here, as well.

Changes introduced in #333:

• The repo is now a package compatible with Unity Package Manager.
• Icons are embedded into a generated class, which reduces clutter in the texture browser.

#333 fixes introduced in this PR:

• The root folder of RealtimeCSG is now found using a directory search, instead of a file search. This removes the need for finding RealtimeCSG.asmdef or a text document.

#334 and #335 fixes:

• UNITY_EDITOR define was added to the define constraints of RealtimeCSG.asmdef, and compatible platforms were defined, which fixes player builds failing due to editor classes included in build.

Other changes:

• Readme updated to include information on how to import the project with supported methods, and formatting adjusted to be easier to read.
• Included information on how to contribute to the project
• Included changes from #341

[LogicalError]

Sorry made some tiny modifications to 2 files, didn't notice your PR until after that

[nukeandbeans]

Sorry made some tiny modifications to 2 files, didn't notice your PR until after that

No worries! I'll just bring the changes over when I have time.

[nukeandbeans]

All merge conflicts were resolved. If all is good, then I'll go ahead and merge the changes + create a release.

[nukeandbeans]

Just occurred to me that there should probably be a FAQs section in the readme. I'll add that in after work later today. Beyond that, that is all the things I can think of that needed fixing or adding, unless someone else brings something up.

[nukeandbeans]

Trying this PR in an actual project seems to cause an extended load time, that seems to go on infinitely (freezes the editor basically). I'm looking into changing how MaterialUtility.CreateRenderPipelineVersionOfDefaultMaterial(Material, string) handles getting paths for things, as it appears that finding directories or files is extremely slow with a lot of folders (sensible). Due to this, I'm going to mark this PR as a draft until I can fully fix this problem. Additionally, it appears to not work as a local package (cannot find folder), and I'm hoping that fixing the above will also fix that.

[nukeandbeans]

Farther digging provides that the hang is related to render pipelines and material generation. Looking into how to fix this.

[nukeandbeans]

Last commit should hopefully fix the aforementioned material generation issue and allow RCSG to work with RPs properly, again.